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

Chapter 12
The vice-regents of God

330. 1. Imam Ali ar-Reza (p): The Imams are the vice-regents of God on earth.

331. 3. Imam Jafar as-Sadiq (p) was asked regarding the words of God: God has promised those of you who believe and do righteous deeds that He will surely make them successors on earth, as those before them... (Q, 24:55)
The Imam (p): They are the Imams (p).