Sahih Bukhari, Chapter: Times of the Prayers

Allah's Apostle said, "Angels come to you in succession by night and day and all of them get together at the time of the Fajr and 'Asr prayers. Those who have passed the night with you (or stayed with you) ascend (to the Heaven) and Allah asks them, though He knows everything about you, well, "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."