Jaʿfar ibn Muḥammad al-Ṣādiq (Arabic: جعفر بن محمد الصادق; 700 or 702–765 C.E.), commonly known as Jaʿfar al-Sadiq or simply al-Sadiq (The Truthful)
Is the sixth Shia Imamand major figure in the Hanafi and Maliki schools of Sunni jurisprudence. He was a descendant of Ali on the side of his father, Muhammad al-Baqir, and of Muhammad ibn Abu Bakron the side of his mother, Umm Farwah bint al-Qasim. Muhammad ibn Abu Bakr was raised by Ali, but was not his son. Ali used to say: "Muhammad Ibn Abu Bakr is my son but from Abu Bakr's lineage". Al-Sadiq is the last individual to be recognized by all Shia sects as an Imam (except the Zaydiyyah), and is revered by Sunni Muslims as a transmitter of Hadith and a prominent jurist.
Al-Sadiq was born in either 700 or 702 CE. He inherited the position of imam from his father in his mid-thirties. As imam, al-Sadiq stayed out of the political conflicts that embroiled the region, evading the many requests for support that he received from rebels. He was the victim of some harassment by the Abbasid caliphs, and was eventually, according to most Shia Muslims, poisoned at the orders of the Caliph al-Mansur.
He was a significant figure in the formulation of Shia doctrine. The traditions recorded from al-Sadiq are said to be more numerous than all hadiths recorded from all other Shia imams combined. As the founder of "Ja'fari jurisprudence", al-Sadiq also elaborated the doctrine of Nass (divinely inspired designation of each imam by the previous imam), and Ismah (the infallibility of the imams), as well as that of Taqiyyah.
The question of succession after al-Sadiq's death was the cause of division among Shias who considered his eldest son, Isma'il (who had died before his father) to be the next imam, and those who believed his third son Musa al-Kadhim was the imam. The first group became known as the Ismailis and the second, larger, group was named Ja'fari or the Twelvers.