Sahih Muslim Book 35, Hadith Number 6505.
Abu Huraira reported Allah’s Apostle (may peace be upon him) as saying Allah has mobile (squads) of angels, who have no other work (to attend to but) to follow the assemblies of Dhikr and when they find such assemblies in which there is Dhikr (of Allah) they sit in them and some of them surround the others with their wings till the space between them and the sky of the world is fully covered, and when they disperse (after the assembly of Dhikr is adjourned) they go upward to the heaven and Allah, the Exalted and Glorious, asks them although He is best informed about them: Where have you come from? They say: We come from Thine servants upon the earth who had been glorifying Thee (reciting Subhan Allah), uttering Thine Greatness (saying Allah o-Akbar) and uttering Thine Oneness (La ilaha ill Allah) and praising Thee (uttering al-Hamdu Lillah) and begging of Thee. Be would say: What do they beg of Me? They would say: They beg of Thee the Paradise of Thine. He (God) would say: Have they seen My Paradise? They said: No, our Lord. He would say: (What it would be then) if they were to see Mine Paradise? They (the angels) said: They seek Thine protection. He (the Lord) would say: Against what do they seek protection of Mine? They (the angels) would say: Our Lord, from the Hell-Fire. He (the Lord) would say: Have they seen My Fire? They would say: No. He (the Lord) would say: What it would be if they were to see My Fire? They would say: They beg of Thee forgiveness. He would say: I grant pardon to them, and confer upon them what they ask for and grant them protection against which they seek protection. They (the angels) would again say: Our Lord, there is one amongst them such and such simple servant who happened to pass by (that assembly) and sat there along with them (who had been participating in that assembly). He (the Lord) would say: I also grant him pardon, for they are a people the seat-fellows of whom are in no way unfortunate.