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

Chapter 13
The beacons of God

332. 1. Imam Muhammad al-Baqir (p) commenting on the verse of the Noble Quran: Believe, therefore in God and His Messenger and in the Light that We have sent down… (Q, 64:8)
The Imam (p): The light that has been sent down are the Infallible Imams (p), the descendants of Prophet Muhammad (p)…

333. 2. In the Noble Quran God says: Those who follow the Apostle, the unlettered Prophet, whom they find mentioned in their own (scriptures), the Torah and the Gospel and he commands them in what is just, and forbids them from what is evil; he allows them all that is good, and prohibits them from what is bad; He releases them from their heavy burdens and from the yokes that are upon them. So it is those who believe in him, honour him, help him, and follow the Light which is sent down with him, it is they who will prosper. (Q, 7:157)
Imam Jafar as-Sadiq (p) commenting on the above verse: Light meant Imam Ali (p) and the other Imams (p).

334. 6. Muhammad Ibn al-Fudayl: I asked Imam Ali ar-Reza (p) about the words of God, the Blessed, the Sublime: They desire to extinguish with their mouths the Light of God… (Q, 61:8)
Imam Ali ar-Reza (p): They desire to extinguish with their mouths the mastership (Wilayat) of Imam Ali (p).
Fudayl: I asked about the words of God, the Sublime: …God will perfect His Light… (Q, 61:8)
The Imam (p): God says that He will perfect the Imamate, and the Imamate is the Light. These are the words of God.
Fudayl: And God says: Believe in God and His Messenger, and in the Light which We have sent down… (Q, 64:8).
The Imam (p): The Light is the Imam (p).