الكافي 1/4/72/7

7. Ahmad ibn Mihran has narrated from Muhammad ibn Ali from Muhammad ibn Fudayl who has said that al-Makhdhumi whose mother was one of the children of Ja‘far ibn abu Talib has narrated the following. “Abu al-Hassan Musa عليه السلام once called all of us to see him. We all gathered and then he said to us, “Do you know why have I called you?” We said, “We do not know.” He then said, “Bear testimony that this my son is the executor of my will, the director of my affairs and the succeeding Imam عليه السلام . Whoever has a loan due on me should demand from my son, this one. To whoever I may have promised anything should also acquire from him. Whoever must see me must not come to see me with writing from him.”