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

Chapter 43
Knowledge of Imams grows

386. 1. Imam Jafar as-Sadiq (p): If our knowledge was not increased, we would be finished.

387. 4. Imam Jafar as-Sadiq (p): The knowledge that comes from God, to whom belong the Might and Majesty, is first given to the Prophet Muhammad (p), then to Imam Ali (p), then to us, one after the other, and the last of us may not be more knowledgeable than the first of us.