Sahih Al Bukhari Hadith Number 781

Narrated Abu Huraira: Allah’s Messenger (peace be upon him) said, “If any one of you says, “Amin” and the angels in the heavens say “Amin” and the former coincides with the latter, all his past sins will be forgiven.”

10  Call to Prayers (Adhaan)
Hadith number 781
Volume no 1
Chapter: 112 (Superiority of saying Amin)