I asked Abu Abdullah (a): “Allah may make me your sacrifice. What is the meaning of the saying that I have heard from you?”
The Imam (a) asked: “Which one?”
I said: “(It is your saying) the believer sees through the light of Allah.”
He said: “O Muawiya, Allah created the believers from His light, made them in His mercy, and put them under the obligation of being loyal to us when He introduced Himself to them and they acknowledged Him. Hence, the believers are the brothers of each other. Their father is the light and their mother is the mercy. They thus see through that light from which they were created.

Source: Fadha’il Al-Shi’a, Sh. Al-Saduq, Hadith #21