Sahih Bukhari
Oneness, Uniqueness of Allah (Tawheed)
Volume: 009 / Book Number: 093 / Hadith Number: 578
Narrated Abu Huraira:
Allah's Apostle said, "There are angels coming to you in succession at night, and others during the day, and they all gather at the time of 'Asr and Fajr prayers. Then the angels who have stayed with you overnight ascend (to the heaven) and He (Allah) asks them though He perfectly knows their affairs. 'In what state have you left my slaves?' They say, 'When we left them, they were praying and when we came to them they were praying.' "