Hadith of two weighty things (i.e. Hadith Thaqalayn) is one of the traditions documented in both the Sunni and the Shia Hadith collections. However, in the Sahih of Imam Muslim, Zayd Ibn Arqam was quoted thus:
…”ثم قال قام رسول الله صلى الله عليه وسلم يوما فينا خطيبا بماء يدعى خما بين مكة والمدينة فحمد الله وأثنى عليه ووعظ وذكر ثم قال أما بعد ألا أيها الناس فإنما أنا بشر يوشك أن يأتي رسول ربي فأجيب وأنا تارك فيكم ثقلين أولهما كِتَابُ اللَّهِ فيه الهدى والنور فخذوا بكتاب الله واستمسكوا به فحث على كتاب الله ورغب فيه ثم قال وَأَهْلُ بَيْتِي أذكركم الله في أهل بيتي أذكركم الله في أهل بيتي أذكركم الله في أهل بيتي"
"…Then, he said: "One day, the Messenger of Allāh, peace be upon him, stood and addressed us at a watering place called Khum, between Makkah and Medina. He praised and glorified Allāh, and he exhorted and reminded us. Then, he said: ‘O mankind! I am only human. The messenger of my Lord (i.e. the angel of death) will soon reach me and I will answer the call (of death). But, I am leaving among you Two Weighty Things. The first of them is the Book of Allāh, in which is guidance and light. So hold fast to the Book of Allāh and adhere to it.’ And he encouraged us to adhere to the Book of Allāh. Then, he said: ‘And my Ahl al-Bayt. I remind you of Allāh with regard to my Ahl al-Bayt! I remind you of Allāh with regard to my Ahl al-Bayt! I remind you of Allāh with regard to my Ahl al-Bayt!’”
Imām Aḥmad (d. 241 H) has recorded this same exact report, and Shaykh al-Arnaūṭ has this comment about it:
إسناده صحيح على شرط مسلم
Its chain is ṣaḥīḥ upon the standard of Muslim.
So, there is no doubt about the authenticity of the narration. The key highlights of this hadith are:
1. The Prophet left only two things among his Ummah until the Day of Resurrection – not more, not less.
2. The Prophet (PBUH) referred to both of these things "Two Weighty Things” (i.e. Thaqalayn).
3. These two weighty things are the Book of Allāh and the Ahl al-Bayt of the Prophet (PBUH).
4. The Messenger conjoined the Qur’ān and his Ahl al-Bayt with the coordinating conjunction "and”
5. Then, he (PBUH) ordered the Muslims repeatedly to remember Allāh concerning his Ahl al-Bayt.
Meanwhile, the first question that comes to mind is: "What did the Prophet mean when he referred to the Qur’an and the Ahl al-Bayt as "Two Weighty Things”?
Imām Ibn al-Athīr (d. 606 H), the leading Sunnī ḥadīth semanticist, explains:
)ثقل) (هـ) فيه :إني تارك فيكم الثقلين :كتاب الله وعترتي سماهما ثقلين ; لأن الأخذ بهما والعمل بهما ثقيل . ويقال لكل خطير (نفيس) ثقل ، فسماهما ثقلين إعظاما لقدرهما وتفخيما لشأنهما .
(Thaqula): concerning it: "I am leaving among you the Two Weighty Things (al-thaqalayn): the Book of Allāh and my nearest kinsfolk.” He named them both thaqalayn because holding fast to both of them and following them are weighty. Every weighty, (priceless) thing is called a ‘Thaqal’. Therefore, he named them thaqalayn out of respect for their status and in honour of their weightiness.
Imām al-Nawawī (d. 676 H) also says:
قوله صلى الله عليه و سلم (و أنا تارك فيكم ثقلين( فذكر كتاب الله واهل بيته. قال العلماء سميا ثقلين لعظمهما وكبير شأنهما وقيل لثقل العمل بهما
His statement, peace be upon him: (I am leaving among you Two Weighty Things) and he mentioned the Book of Allāh and his Ahl al-Bayt. The ‘ulamā said: "He named them thaqalayn due to their significance and the greatness of their weightiness.” It is also said: "Due to the weightiness of following both of them.”
Imām Abū Manṣūr al-Azharī (d. 370 H), a top classical Sunni lexicographer, states:
روى عن النبي صلى الله عليه وسلم أنه قال في مرضه الذي مات فيه: إني تارك فيكم الثقلين: كتاب الله وعترتي، ولن يفترقا حتى يردا على الحوض، فسر النبي صلى الله عليه الثقلين فجعلهما كتاب الله. جل وعز وعترته عليه السلام؛ وقد فسرت العترة فيما تقدم ]وهم[ جماعة عشيرته الأدنون. وقال أبو العباس أحمد بن يحيى: سميا ثقلين لأن الأخذ بهما ثقيل، والعمل بهما ثقيل. وأصل الثقل ان العرب تقول لكل شيء نفيس مصون: ثقل
It is narrated that the Prophet (peace be upon him) said during his fatal illness: "I am leaving among you the Two Weighty Things: the Book of Allāh and my nearest kinsfolk. Both shall never separate until they meet me at the Lake-Fount.” So, the Prophet (peace be upon him) identified the Two Weighty Things as the Book of Allāh, the Almighty, the Most Glorious, and his nearest kinsfolk, peace be upon him. I have already explained what the word al-‘itrah means, and [they] are his nearest kinsfolk. Abū al-‘Abbās Aḥmad b. Yaḥyā said: "They were named thaqalayn because holding fast to both of them is weighty, and following them both is also weighty. The root of al-thaqal is that the Arabs call anything which is priceless and well-guarded: thaqal.
And Imām Ibn Manẓūr (d. 711 H), the legendary classical Sunni lexicographer, has the same submission:
التهذيب: وروي عن النبي صلى الله عليه وسلم أنه قال في آخر عمره :إني تارك فيكم الثقلين: كتاب الله وعترتي، فجعلهما كتاب الله عز وجل وعترته، وقد تقدم ذكر العترة. وقال ثعلب :سميا ثقلين لأن الأخذ بهما ثقيل، والعمل بهما ثقيل، قال: وأصل الثقل أن العرب تقول لكل شيء نفيس خطير مصون ثقل، فسماهما ثقلين إعظاما لقدرهما، وتفخيما لشأنهما
Al-Tahdhīb : It is narrated that the Prophet (peace be upon him) said at the end of his lifetime: "I am leaving among you the Two Weighty Things: the Book of Allāh and my nearest kinsfolk.” So, he identified them as the Book of Allāh, the Almighty, the Most Glorious, and his nearest kinsfolk. Al-‘itrah has already been mentioned. Tha’lab said: "They were named thaqalayn because holding fast to them both is weighty, and following them both is weighty.” He said: "The root of al-thaqal is that the Arabs call anything which is priceless, weighty and well-guarded: thaqal.” Therefore, he named them thaqalayn out of respect for their status, and in honour of their weightiness.
Obviously, three significant points are established exclusively through the Prophet’s naming of the Qur’an and the ‘Ahl al-Bayt as thaqalayn:
1. Allāh and His Messenger greatly value both of them, and consider them priceless.
2. Holding fast to the two is weighty in the sight of Allāh and His Messenger.
3. Following both of them is weighty in the sight of Allāh and His Messenger.
Besides, the Prophet further conjoined the Qur’ān and his Ahl al-Bayt with the coordinating conjunction: "and” (wa). What is the direct implication of this? Imām al-Sarakhsī (d. 490 H), a major classical Sunni jurist, states about conjunction:
الأصل فيه الواو فلا خلاف أنه للعطف (ولكن عندنا هو للعطف) مطلقا فيكون موجبه الاشتراك بين المعطوف والمعطوف عليه في الخبر من غير أن يقتضي مقارنة أو ترتيبا وهو قول أكثر أهل اللغة
The default conjunction is "and” (wa). There is no disagreement about the fact that it is for conjunction (however, in our view, it is for conjunction) absolutely. Therefore, it necessarily creates partnership between the two conjuncts in the information (given), without necessarily indicating equality or hierarchy (between them). And this is the view of the majority of linguists.
So, by using "and” (i.e. ‘wa’) to conjoin "the Book of Allāh” and "my Ahl al-Bayt,” the Messenger of Allāh is:
(i) necessarily indicating that the phrase "my Ahl al-Bayt” is a second conjunct;
(ii) necessarily creating partnership between the Qur’ān and the Ahl al-Bayt in the information given;
(iii) necessarily affirming for the Ahl al-Bayt what he affirmed for the Qur’ān;
(iv) necessarily giving in favour of the Ahl al-Bayt the same ruling which he gave in favour of the Qur’ān in the hadith; and
(v) not necessarily indicating that they are equal or different in rank.
Then, what did he affirm for the first conjunct – the holy Qur’ān:
أنا تارك فيكم ثقلين أولهما كِتَابُ اللَّهِ فيه الهدى والنور فخذوا بكتاب الله واستمسكوا به فحث على كتاب الله ورغب فيه ثم قال وَأَهْلُ بَيْتِي
"I am leaving among you Two Weighty Things. The first of them is the Book of Allāh, in which is guidance and light. So hold fast to the Book of Allāh and adhere to it.” And he encouraged us to adhere to the Book of Allāh. Then, he said: "And my Ahl al-Bayt.”
In other words:
(a) There is guidance in the Ahl al-Bayt too;
(b) There is light in them as well;
(c) It is likewise obligatory to hold fast to them; and
(d) It is similarly obligatory to adhere to them.
These four facts were conveyed by the Prophet (PBUH) through that singular "and” (wa) which he mentioned before "my Ahl al-Bayt.”
This then takes us to this narration of Imām al-Ṭaḥāwī (d. 321 H):
حدثنا إبراهيم بن مرزوق قال : حدثنا أبو عامر العقدي قال : حدثنا كثير بن زيد، عن محمد بن عمر بن علي ، عن أبيه ، عن علي ، أن النبي صلى الله عليه وسلم حضر الشجرة بخم فخرج آخذا بيد علي فقال :يا أيها الناس، ألستم تشهدون أن الله عز وجل ربكم؟ قالوا: بلى، قال: ألستم تشهدون أن الله ورسوله أولى بكم من أنفسكم، وأن الله عز وجل ورسوله مولياكم؟ قالوا: بلى، قال: فمن كنت مولاه فإن هذا مولاه، أو قال: فإن عليا مولاه - شك ابن مرزوق - إني قد تركت فيكم ما إن أخذتم به لن تضلوا :كتاب الله سببه بأيديكم، وأهل بيتي.
Ibrāhīm b. Marzūq - Abū ‘Āmir al-‘Aqadī – Kathīr b. Zayd – Muḥammad b. ‘Umar b. ‘Alī – his father - ‘Alī: Verily, the Prophet, peace be upon him, came to a tree at (Ghadīr) Khumm. Then he came out, holding the hand of ‘Alī, and saying: "O mankind! Do you not testify that Allāh the Almighty is your Lord?” They said, "Yes, we do.” He said, "Do you not testify that Allāh and His Messenger are more entitled to you than you are to yourselves and that Allāh the Almighty and His Messenger are your Master?” They said, "Yes, we do”. He said, "So, whosoever Allāh and His Messenger are his Master, verily this one – or ‘Alī - is his master. I have left among you that which if you hold fast to it you will never go astray: the Book of Allāh – one end of which is in your hands – and my Ahl al-Bayt.”
Its chain is ḥasan.
Here, the Messenger (PBUH) has again conjoined the Qur’ān with the Ahl al-Bayt with "and” (wa). Therefore, he deliberately affirmed for the Ahl al-Bayt what he affirmed for the Book of Allāh with that coordinating conjunction. Thus, the Ahl al-Bayt are those that must be followed too – like the Qur’ān – in order to be rightly guided. Moreover, just as it is impossible to remain on the Right Path while abandoning the Book of Allāh. Similarly, it is equally impossible to be upon the correct guidance of Allāh and His Messenger while abandoning the Ahl al-Bayt.
Then, let us further pay close attention to the format of the text of Ḥadīth al-Thaqalayn. If someone says: "you will succeed if you work hard” what does he mean? He obviously means: if you do not work hard, you will not succeed. It is the same case if he says: "You will be rightly guided if you follow the Qur’ān.” In other words, if you do not follow the Book of Allāh, you will not be guided. Now, compare these examples with this report of Imām al-Tirmidhī (d. 279 H):
حدثنا نصر بن عبد الرحمن الكوفي حدثنا زيد بن الحسن هو الأنماطي عن جعفر بن محمد عن أبيه عن جابر بن عبد الله قال رأيت رسول الله صلى الله عليه و سلم في حجته يوم عرفة وهو على ناقته القصواء يخطب فسمعته يقول يا أيها الناس إني قد تركت فيكم ما إن أخذتم به لن تضلوا كتاب الله وعترتي أهل بيتي
Naṣr b. ‘Abd al-Raḥman al-Kūfī – Zayd b. al-Ḥasan al-Anmāṭī – Ja’far b. Muḥammad – his father – Jābir b. ‘Abd Allāh: I saw the Messenger of Allāh, peace be upon him, during his Ḥajj on the Day of ‘Arafat. He was upon his camel, al-Qaṣwā, giving a khuṭbah, and I heard him saying: "O mankind! I have left among you that which if you hold fast to it, you will never go astray: the Book of Allāh and my nearest kinsfolk, my Ahl al-Bayt.”
وهذا حديث حسن غريب من هذا الوجه
And this ḥadīth is ḥasan gharīb (i.e. has a ḥasan chain) from this route.
Al-Albānī says too:
Thus, if one does not hold fast to the Ahl al-Bayt alongside the Qur’ān, one will definitely go astray. The text is explicit, clear and direct. In view of this, if one wants to know whether he is on the right path of Allāh or not, one needs himself: "Am I following the Book of my Lord and the Ahl al-Bayt of His Prophet together in my religion?” If one cannot give a resounding "Yes” answer to this question, then one is surely not on the straight path. One merely belongs to one of the misguided sects.
The Qur’ān itself also has the same verdict concerning those who disobey the Prophet’s command, in Ḥadīth al-Thaqalayn, to follow his Ahl al-Bayt after him:
وَمَا كَانَ لِمُؤْمِنٍ وَلَا مُؤْمِنَةٍ إِذَا قَضَى اللَّهُ وَرَسُولُهُ أَمْرًا أَن يَكُونَ لَهُمُ الْخِيَرَةُ مِنْ أَمْرِهِمْ وَمَن يَعْصِ اللَّهَ وَرَسُولَهُ فَقَدْ ضَلَّ ضَلَالًا مُّبِينًا
When Allāh and His Messenger have decided something, it is not for any man or woman of the believer to have a choice about it. Anyone who disobeys Allāh and His Messenger is clearly misguided.
وَمَن يَعْصِ اللَّهَ وَرَسُولَهُ فَإِنَّ لَهُ نَارَ جَهَنَّمَ خَالِدِينَ فِيهَا أَبَدًا
As for him who disobeys Allāh and His Messenger, he will have the Fire of Hell, remaining in it timelessly, for ever and ever.
 Abū al-Ḥusayn Muslim b. al-Ḥajjāj al-Qushayrī al-Naysābūrī, Ṣaḥīḥ Muslim (Beirut: Dār Iḥyā al-Turāth al-‘Arabī) [annotator: Muḥammad Fuād ‘Abd al-Bāqī], vol. 4, p. 1873, # 2408 (36); Imam Abul Hussain Muslim Ibn al-Hajjaj, English Translation of Sahih Muslim (Riyadh: Darussalam: 2007 CE) [annotator: Abu Tahir Zubair ‘Ali Za’i; translator: Nasiruddin al-Khattab; final review: Abu Khaliyl], vol. 6, pp. 267-268, # 2408 (36) 
 Abū ‘Abd Allāh Aḥmad b. Ḥanbal al-Shaybānī, Musnad (Cairo: Muasassat Qurṭubah) [annotator: Shu’ayb al-Arnaūṭ], vol. 4, p. 366, # 19285
 Majd al-Dīn Abū al-Sa’ādāt al-Mubārak b. Muḥammad al-Jazarī b. al-Athīr, al-Nihāyah fī Gharīb al-Ḥadīth wa al-Athar (Al-Dammām: Dār Ibn al-Jawzī; 1st edition, 1421 H) [annotator: ‘Alī b. Ḥasan b. ‘Alī b. ‘Abd al-Ḥamīd al-Ḥalabī al-Atharī], p. 125
 Abū Zakariyā Yaḥyā b. Sharaf al-Nawawī, Ṣaḥīḥ Muslim bi Sharḥ al-Nawawī (Muasassat Qurṭubah; 2nd edition, 1414 H), vol. 15, p. 256, # 2408 (36)
 Abū Manṣūr Muḥammad b. Aḥmad al-Azharī, Tahdhīb al-Lughah (Dar al-Maṣriyyah) [annotator: Prof. ‘Abd al-Salām Hārūn], vol. 9, p. 78
 Abū al-Faḍl Jamāl al-Dīn Muḥammad b. Mukarram b. Manẓūr al-Afrīqī al-Miṣrī, Lisān al-‘Arab (Beirut: Dār Ṣādir), vol. 11, p. 88
 Abū Bakr Muḥammad b. Aḥmad b. Abī Sahl al-Sarakhsī, Uṣūl al-Sarakhsī (Hyderabad; Lajnat Iḥyā al-Ma’ārif al-‘Uthmāniyyah; 1414 H) [annotator: Abū al-Wafā al-Afghānī], vol. 1, p. 200
 Abū Ja’far Aḥmad b. Muḥammad b. Salāmah al-Ṭaḥāwī, Sharḥ Mushkil al-Athār (Beirut: Muasassat al-Risālah; 1st edition, 1415 H) [annotator: Shu’ayb al-Arnaūṭ], vol. 5, p. 13, # 1760
 Muḥammad b. ‘Īsā b. Sawrat al-Tirmidhī, Ṣaḥīḥ Sunan al-Tirmidhī (Riyadh: Maktabat al-Ma’ārif; 1st edition, 1420 H) [annotator: Muḥammad Nāṣir al-Dīn al-Albānī], vol. 3, pp. 542-543, # 3786
 Ibid, vol. 3, p. 543, # 3786
 Qur’ān 33:36
 Qur’ān 72:23