First and foremost, it is a fundamental doctrine of Shi’a Islam that Muḥammad, peace be upon him and his family, was the last and final Prophet and Messenger of Allāh. Thus, the belief of the Shi’ites is that there is absolutely no prophet or messenger after the Prophet Muhammad (PBUH). He was the last but the best of all the Prophets (AS) and Messengers (AS) of God. In this regards, Shaykh al-Kulaynī records:
Muḥammad b. Yaḥyā al-Ash’arī – Aḥmad b. Muḥammad – al-Barqī – al-Naḍr b. Suwayd – Yaḥyā b. ‘Imrān al-Ḥalabī – Ayyūb b. al-Ḥurr:
I heard Abū ‘Abd Allāh, peace be upon him, saying: "Verily, Allāh, honoured be His Mention, ended the prophets with your Prophet. So, there will never be any prophet after him. He also ended the scriptures with your Scripture. So, there will be never be any scripture after it (i.e. its revelation). He revealed in it explanations of all things, and (explanations of) your creation and the creation of the heavens and the earth, and news of what occurred before you and the settlement of your disputes, and news of what will happen after you, and the affairs of the Paradise and the Hellfire, and that to which you are proceeding.”[1]
Moreover, the teachings of all the twelve Imams, in all situations and circumstances, were merely transmissions of the authentic Sunnah of Muḥammad. In another words, the twelve Imams are only the interpreters and transmitters of the sayings and practices of the Holy Prophet (PBUH). In view of this, therefore, the sayings and practices of the Imams are in conformity with the sayings and practices of the Holy Prophet (PBUH). Meanwhile, there are several mutawātir narrations in the Shi’a Hadith collections to support the claim.
Imam Muhammad Baqir (AS) was reported saying:
"O Jābir! I swear by Allāh, if we speak to people with our opinions, we will then surely be among those who will perish. However, we speak to them with reports, which are with us, from the Messenger of Allāh, peace be upon him and his family. We inherit them from generation to generation. We treasure them as these people treasure their gold and silver.”[2]
Likewise, Abu al-Maghra Sama’ah reported thus:
I said to Abū al-Ḥasan Mūsā, peace be upon him, "Is everything in the Book of Allāh and Sunnah of His Prophet, peace be upon him and his family? Or do you express your opinions also?” He said, "Rather, EVERYTHING is in the Book of Allāh and the Sunnah of His Prophet, peace be upon him and his family.”[3]
Imam Ja’far Sadiq (AS) was also reported saying:
"My ḥadīth is the ḥadīth of my father. The ḥadīth of my father is the ḥadīth of my grandfather. The ḥadīth of my grandfather is the ḥadīth of al-Ḥusayn. The ḥadīth of al-Ḥusayn is the ḥadīth of al-Ḥasan. The ḥadīth of al-Ḥasan is the ḥadīth of Amīr al-Mūminīn, peace be upon him. The ḥadīth of Amīr al-Mūminīn is the ḥadīth of the Messenger of Allāh, peace be upon him and his family. The ḥadīth of the Messenger of Allāh is the Statement of Allāh, the Almighty, the Most Glorious.[4]
In light of the above narrations and the likes, it is established that the sayings and practices of the twelve Imams without any exceptions are sayings and practices of the Holy Prophet (PBUH) himself. In addition, for the fact that the term "Shi’a” was used for the first time by the Holy Prophet (AS) himself for a selected group who were partisans of Ali Ibn Abu-Talib among his companions but it is used at present for those Muslims who believe that the leadership of the community after Muhammad belongs to Ali and his successors. In another word, those who believe that Muhammad had designated Ali as his heir, Imam and Caliph of the Muslim immediately after him and that the authority never goes out of his descendants are called Shia. Interestingly, those who love and follow the pure progeny (i.e. Ahlul-Bayt) of the Prophet are in reality the lovers and followers of the Holy Prophet (PBUH). Consequently, the Shi’a of Ali are indeed the Shi’a of Muhammad in literal sense. In a nutshell, the Shī’ah of any of the twelve Imams are none but Shī’ah of the Holy Prophet (PBUH).
[1] Abū Ja’far Muḥammad b. Ya’qūb b. Isḥāq al-Kulaynī al-Rāzī, al-Uṣūl min al-Kāfī (Tehran: Dār al-Kutub al-Islāmiyyah; 3rd edition) [annotator: ‘Alī Akbar al-Ghiffārī], vol. 1, p. 269, # 3
[2] Abū Ja’far Muḥammad b. al-Ḥasan b. Farūkh al-Ṣaffār, Baṣāir al-Darajāt al-Kubrā fī Faḍāil Āl Muḥammad (Tehran: Muasassat al-A’lamī; 1404 H), p. 320, # 6
[3] Abū Ja’far Muḥammad b. Ya’qūb b. Isḥāq al-Kulaynī al-Rāzī, al-Uṣūl min al-Kāfī (Tehran: Dār al-Kutub al-Islāmiyyah; 3rd edition) [annotator: ‘Alī Akbar al-Ghiffārī], vol. 1, p. 62, # 10
[4] Abū Ja’far Muḥammad b. Ya’qūb b. Isḥāq al-Kulaynī al-Rāzī, al-Uṣūl min al-Kāfī (Tehran: Dār al-Kutub al-Islāmiyyah; 3rd edition) [annotator: ‘Alī Akbar al-Ghiffārī], vol. 1, p. 53, # 14