Angels Visit The Believer Every Night and Every Day

Allah’s messenger (S) said, 

"There are angels who take turns in visiting you by night and by day, and they all assemble at the dawn (Fajr) and the afternoon (`Asr) prayers. Those who have spent the night with you, ascend to the heaven and their Rubb, Who knows better about them, asks: `In what condition did you leave My slaves?' They reply: `We left them while they were performing prayer and we went to them while they were performing prayer.'''

[Al-Bukhari and Muslim]

Commentary: The angels for the night come at the time of `Asr when the angels for the morning are present. This is how the angels of the two shifts assemble at this time. The angels of the shift of `Asr leave their duty in the morning, and the angels of the morning shift resume their duty when the pious persons are engaged in Fajr prayer. This is how the two groups assemble again at that time. Thus, when the angels come or go, the people who are punctual in their prayer are engaged in Fajr and `Asr. Almighty Allah knows everything but even then He asks the angels about his pious slaves so that the piousness of the believers and their merit and distinction become evident to them.

Read more hadiths about the excellence of Fajr and Asr prayers: http://www.witness-pioneer.org/vil/hadeeth/riyad/08/chap188.htm