Sahih Bukhari Volume 9, Book 93, Hadith Number 525A.

Narrated By Abu Huraira : Allah’s Apostle said, “(A group of) angels stay with you at night and (another group of) angels by daytime, and both groups gather at the time of the ‘Asr and Fajr prayers. Then those angels who have stayed with you overnight, ascend (to Heaven) and Allah asks them (about you)… and He knows everything about you. “In what state did you leave My slaves?’ The angels reply, ‘When we left them, they were praying, and when we reached them they were praying.'”