Al-Sadiq Ja'far b. Muhammad b. Ali b. al-Husayn, peace be on them, was out of all his brothers (the one who was) the successor (khailfa) of his father, Muhammad b. Ali, peace be on them, his testamentary trustee (was'i), who was in charge of the Imamate (al- qa'im bi-al-imama) after him. He stood out among their group for his great merit (fadl); he was the most celebrated, the greatest in rank and the most illustrious of them in (the eyes) of both the non-Shia (al-amma) and the Shi'a (al-khassa). The people transmitted on his authority the religious sciences which travellers carried with them (around many countries) and thus his fame was spread throughout the lands. The learned scholars have transmitted on the authority of no other member of the House (ahl al-bayt) as much as they have transmitted on his authority. None of them met as many of the reporters of traditions (ahl al-athar wa-naqalat al-akhbar) as he did, nor did the latter transmit on their authority to the same extent as they transmitted on the authority of Ab-u Abd Allah (Ja'far b. Muhammad), peace be on him. The specialists in tradition (ashab al- hadith) have gathered together the names of those who narrated on his authority, who were reliable despite differences in views and doctrines and they were four thousand men. The clear evidence for his Imamate, peace be on him, was such that it overcame (men's) hearts and silenced (the attempts of) an opponent to denigrate it with doubts.
He was born in Medina in the year 83 A.H. (702) and he, peace be on him, died in (the month of) Shawwal in the year 148 A.H. (765) at the age of sixty-five. He was buried in (the cemetery of) al-Baqi' alongside his father, his grandfather and his (great-great) uncle, al- Hasan, peace be on them. His mother was Umm Farwa, the daughter of al-Qasim b. Muhammad b. Abi Bakr. His Imamate, peace be on him, lasted for thirty-four years. His father, Abu Jafar (Muhammad b. Ali), peace be on him, clearly gave him the trusteeship (of the Imamate) and gave him an explicit designation (nass jali) for the Imamate.