Maliks Muwatta Book 9, Hadith Number 85.
Section : Prayer in General.
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.'”