Usul al-Kafi
Book 4:
The Book of Divine Proof
Chapter 74
Imamate of Imam Ali an-Naqi (p): the tenth Imam
436. 1. Imam Muhammad at-Taqi (p) was asked: …Who will be the next Imam after you?
The Imam (p): My son Imam Ali an-Naqi (p) will be my successor in Imamate.