Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this same chain (it is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him) that the Messenger of Allah, peace and blessings be upon him, said: "Angels come and go among you by night and by day, and they gather at the Fajr prayer and the Asr prayer. Then those who have spent the night among you ascend (in the morning), and (Allah the Exalted) asks them, though He knows best about them: 'In what state did you leave My servants?' They say: 'We left them while they were praying, and when we came to them, they were praying as well.'"