Musnad Abu Ya'la al-Mawsili 159:94

And with his chain of narration, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The angels take turns among you; angels by night and angels by day. They gather at the Asr prayer and the Fajr prayer. Then those who stayed among you ascend, 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.'"