Usul al-Kafi
Book 4:
The Book of Divine Proof

Chapter 72
Imamate of Imam Ali ar-Reza (p): the eighth Imam

433. 3. Dawood ar-Raqqi reports that he said to Imam Musa al-Kazim (p) that he was getting old and pleaded with the Imam (p) to save him from fire (i.e. from the danger of not knowing the Imam after him). The Imam (p) pointed to his son Imam Ali ar-Reza (p), and said: He will be your master after me.

434. 6. Ziyad Ibn Marwan al-Qandi reports that once he visited Imam Musa al-Kazim (p) and with him was his son, Abul Hasan (p). The Imam (p) said: O Ziyad, this is my son. His writing is my writing, his speech is my speech. His message is my message. What he says is true.