Abu Ja'far Muhammad ibn Usman (304 A.H.- 305 A.H.)  He was designated by the written will of his father to succeed him as the representative of Imam Mahdi.  He had a vast collection of books on Law which he had inherited from his father, and which his father had received from Imams. 

During the period that he served as the chief deputy, he took an oath that the Master of the Age was present among the pilgrims of Makkah every year and he sees them although they fail to see him.

As a sign of Abu Ja'far's supernatural intuition it is related that one of his friends found him inscribing a verse of the Quran on a slab of a stone, and in the margins he was cutting the names of the Imams.  His friends asked him what he was doing.  "It is for my grave", he said, and then he told this man when he expected to die.  To others he expressed the same opinion, and the traditions that his prediction proved to be true.  After he had served the Imams for about fifty years, he died in the year 305 A.H.  He was buried bodies the grave of his mother, on the roadside at the Kufa Gate, at the place where his house stood.