Sahih Bukhari Volume 1, Book 9, Hadith 555

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ حَدَّثَنَا مَالِكٌ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏"‏ يَتَعَاقَبُونَ فِيكُمْ مَلاَئِكَةٌ بِاللَّيْلِ وَمَلاَئِكَةٌ بِالنَّهَارِ، وَيَجْتَمِعُونَ فِي صَلاَةِ الْفَجْرِ وَصَلاَةِ الْعَصْرِ، ثُمَّ يَعْرُجُ الَّذِينَ بَاتُوا فِيكُمْ، فَيَسْأَلُهُمْ وَهْوَ أَعْلَمُ بِهِمْ كَيْفَ تَرَكْتُمْ عِبَادِي فَيَقُولُونَ تَرَكْنَاهُمْ وَهُمْ يُصَلُّونَ، وَأَتَيْنَاهُمْ وَهُمْ يُصَلُّونَ ‏"

Narrated By Abu Huraira : 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."