كتاب بدء الخلق
Beginning of Creation
Chapter: The reference to angels
باب ذِكْرِ الْمَلاَئِكَةِ
Sahih al-Bukhari 3223

Narrated Abu Huraira:

The Prophet (ﷺ) 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." If anyone of you says "Amin" (during the Prayer at the end of the recitation of Surat-al-Faitiha), and the angels in Heaven say the same, and the two sayings coincide, all his past sins will be forgiven."

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 3223
In-book reference : Book 59, Hadith 34
USC-MSA web (English) reference : Vol. 4, Book 54, Hadith 446
  (deprecated numbering scheme)