Yahya related to me from Abu'z-Zinad from al-Araj from AbuHurayra that the Messenger of Allah, may Allah bless him and grant himpeace, said, "A group of angels in the night and a group of angels inthe day alternate with each other among you, and gather together atthe time of the asr and fajr prayers. Then those that have spent thenight among you ascend, and He asks them, and He knows best, 'How didyou leave my slaves?' and they say, 'When we left them they werepraying, and when we came to them they were praying.' "
USC-MSA web (English) reference: Book 9, Hadith 85
Arabic reference: Book 9, Hadith 416