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

Chapter 9
Imams are witnesses of God

321. 1. Imam Jafar as-Sadiq (p) commenting on the verse: How will it be when We shall bring a witness from every people and shall bring you (O Muhammad (p)) as a witness against them? (Q, 4:41) said: This verse has especially been revealed in respect of the followers of Prophet Muhammad (p) (the Muslims). In every age there will be an Imam for them from among us, who will be a witness to them and Prophet Muhammad (p) will be a witness for us.

322. 2. Imam Ali ar-Reza (p) was requested to comment on the verse: Can there be any doubt concerning him who bases himself on a clear proof from his Lord and followed by a witness from Him? (Q, 11:17)
Imam Ali ar-Reza (p): Imam Ali (p) is a witness for the Prophethood of Prophet Muhammad (p) and the Prophet Muhammad (p) is a clear proof for his Lord.

323. 5. Imam Ali (p): God, the Glorious, the Elevated, has made us immune from any kind of abomination and has made us infallible and instituted us as witnesses upon His creations and has made us His Hujjat (the proof of His existence) on the earth and has associated us with the Noble Quran. We shall never be separated from the Noble Quran and the Noble Quran shall never be separated from us.