Musnad Ahmad 30:1001

The Messenger of Allah, may Allah bless him and grant him peace, said: "Angels succeed one another among you: angels by night and angels by day." He said: "They gather at the dawn prayer and the afternoon prayer. Then those who stayed among you ascend to Him, and He asks them—while He is more knowing—: 'How did you leave My servants?' They say: 'We left them while they were praying, and we came to them while they were praying.'"