الكافي 1/4/71/7

7. Muhammad ibn Yahya has narrated from Muhammad ibn al-Husayn from ‘Abd al-Rahman ibn abu Najran from ‘Isa ibn ‘Abdallah ibn Muhammad ibn ‘Umar ibn Ali ibn abu Talib عليه السلام who has said the following. “I said to abu ‘Abdallah عليه السلام . “If something would happen (death of Imam), may Allah will not make me experience it, then who must I follow as my Imam?” The narrator has said that he pointed to his son Musa عليه السلام . I then asked, “What if something will happen to Musa عليه السلام then who must I follow as my Imam?” The Imam عليه السلام said, “Follow his sons. Then he said, “In this way for ever.” I then asked, “What if I will not know him and will not know his place?” The Imam عليه السلام said, “Say, ‘O Lord, I love and take as my guardian whoever is left (on earth) as your authority of the sons of the preceding Imam. This will be enough for you if Allah would so will.”