It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Angels come down to you in succession, some angels by day and some by night, and they gather in the Fajr and Asr prayers. Then those who spent the night among you ascend to the Lord of Majesty and Honour, and Allah Almighty asks them, though He knows everything: 'In what state did you leave My servants?' The angels say: 'When we went to them they were praying, and when we left them they were still praying.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2182