Yahya related to me from Abu'z-Zinad from al-Araj from Abu
Hurayra that the Messenger of Allah, may Allah bless him and grant him
peace, said, "A group of angels in the night and a group of angels in
the day alternate with each other among you, and gather together at
the time of the asr and fajr prayers. Then those that have spent the
night among you ascend, and He asks them, and He knows best, 'How did
you leave my slaves?' and they say, 'When we left them they were
praying, and when we came to them they were praying.' "
Muwatta Malik Book 9, Hadith 90