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

Chapter 26
The Noble Quran leads towards the Imam

353. 1. Imam Ali ar-Reza (p) was asked about the words of God: To everyone, We have appointed heirs over that which they leave by parents, kinsmen and those with whom you have sworn allegiance… (Q, 4:33)
The Imam (p): By this He intended the Imams, to whom God has made you swear your allegiance.