The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1320
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: Angels take turns among you by night and by day, and they all assemble at the dawn and afternoon prayers. Those (of the angels) who spend the night among you, then, ascend, and their Lord asks them, though He is the best informed about them: How did you leave My servants? -they say: We left them while they were praying and we came to them while they were praying.