Sahih al-Bukhari 3223
صحيح البخاري#3223SahihNarrated 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, ascend 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-Fatiha), and the angels in Heaven say the same, and the two sayings coincide, all his past sins will be forgiven
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 59:34