Musnad Ahmad 30:3179

I read to Abd al-Rahman: Malik, and Ishaq narrated to us, he said: Malik informed us, from Abu al-Zinad, from al-A’raj, from Abu Hurairah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Angels succeed one another among you by night—and in the hadith of Abd al-Rahman: 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 of them: 'How did you leave My servants?' They say: 'We left them while they were praying, and we came to them while they were praying.'"