Sahih Al Bukhari – Book of Beginning Of Creation

Volumn 004, Book 054, Hadith Number 446. 
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Narated By Abu Huraira RA: The Prophet PBUH 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."