Musnad Ahmad 30:373

Yazid narrated to us, Muhammad informed us, from Musa ibn Yasar, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Verily, Allah has angels who take turns among you: angels of the night and angels of the day. They gather at the dawn prayer and the afternoon prayer. Then those who were among you ascend to Him, and He asks them—while He is more knowing—saying: 'How did you leave My servants?' They say: 'We left them while they were praying, and we came to them while they were praying.'"