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

Chapter 56
The spirit that protects the Imams

406. 1. Commenting on the verse: And thus have We, by Our command, sent inspiration to you: you did not know what was Revelation, and what was Faith… (Q, 42:52) Imam Jafar as-Sadiq (p) said: That Spirit is a creation among the creations of God, the Mighty, the Glorious, which is greater (more majestic) than Jibrael (Gabriel) (p) and Mikael (Michael) (p) (the two Archangels). It kept company with the Prophet Muhammad (p) to keep him informed and to protect him, and after the Prophet (p), it keeps company with the Imams (p).

407. 2. Commenting on the aforementioned verse (Q, 42:52), Imam Jafar as-Sadiq (p): Ever since this spirit has been sent down, it has not gone back to heavens and now it is with us (the Imams (p)).
408. 6. Imam Ali (p) was approached by a man and was asked: Is ‘The Spirit’ the Jibrael (Gabriel) (p) himself?
The Imam (p): ‘Jibrael is an angel and the Spirit is something other than ‘Jibrael.’
The Imam repeated what he had said in order to emphasize its significance.
The Man: You have said something truly important. But no one thinks that the Spirit is something other than ‘Jibrael.’
The Imam (p): You have gone astray and are repeating what the misguided people say. God, the Exalted One, says: (O Prophet Muhammad (p)) The Command of God has come so do not seek to hasten it. Glorious is He and Exalted far above that which they associate with Him. He sends down the angels with the Spirit… (Q, 16:1-2) Therefore, the Spirit is different from the angels, the Mercy of God be unto them.