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

Chapter 73
Imamate of Imam Muhammad at-Taqi (p): the ninth Imam

435. 10. A believer asked Imam Ali ar-Reza (p): … Who will be the Imam after you?
The Imam (p) pointed towards his three years old son Imam Muhammad at-Taqi (p).
The Believer: But he is only three years old.
The Imam (p): That does not matter. Prophet Isa (Jesus) (p) was also the Hujjat (proof) of God at the age of three.