Narrated Abu Huraira: Allah's Apostle 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."
— Sahih Bukhari, Volume 1, Book 12: Characteristics of Prayer, Hadith #748
Narrated Abu Huraira: Allah's Apostle 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."
This hadith is from the book of Characteristics of Prayer in Sahih Bukhari, compiled by Imam al-Bukhari. It is hadith number 748 in book 12 of volume 1.
This hadith is referenced in the following topics: How to Pray.