Sahih Bukhari Volume 4, Book 59, Hadith 3224

حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، حَدَّثَنَا أَبُو الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ الْمَلاَئِكَةُ يَتَعَاقَبُونَ، مَلاَئِكَةٌ بِاللَّيْلِ وَمَلاَئِكَةٌ بِالنَّهَارِ، وَيَجْتَمِعُونَ فِي صَلاَةِ الْفَجْرِ وَالْعَصْرِ، ثُمَّ يَعْرُجُ إِلَيْهِ الَّذِينَ بَاتُوا فِيكُمْ، فَيَسْأَلُهُمْ وَهْوَ أَعْلَمُ، فَيَقُولُ كَيْفَ تَرَكْتُمْ ‏{‏عِبَادِي‏}‏ فَيَقُولُونَ تَرَكْنَاهُمْ يُصَلُّونَ، وَأَتَيْنَاهُمْ يُصَلُّونَ ‏"‏‏

Narrated By Abu Huraira : The Prophet said, "Angels keep on descending from and ascending to the Heaven in turn, some at night and some by daytime, and all of them assemble together at the time of the Fajr and 'Asr prayers. Then those who have stayed with you over-night, ascent unto Allah Who asks them, and He knows the answer better than they, "How have you left My slaves?" They reply, "We have left them praying as we found them praying."