Muhammad ibn Ja'far al-Sadiq, surnamed al-Dībāj, the younger full brother of Musa al-Kazim, and son of Ja'far al-Sadiq appeared in Mecca in the year 200 A.H. / 815 C.E., in the aftermath of the revolt of Abu'l-Saraya, claiming that he was the Awaited Mahdi. Wikipedia