Allah's Messenger (ﷺ) 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 al-Bukhari Book 10, Hadith 176
← Previous Hadith The Prophet (ﷺ) said, "Say Amin" when the Imam says it and if the Amin of any one of you coincides with that of the angels then all his past sins will be forgiven." Ibn Shihab said, "Allah's Messenger (ﷺ) used to Say "Amin."

Sahih al-Bukhari Book 10, Hadith 175 Next Hadith → Allah's Messenger (ﷺ) said, "Say Amen when the Imam says 'Ghairi l-maghdubi `alaihim wala d-daalleen' (not the path of those who earn Your Anger (such as Jews) nor of those who go astray (such as Christians)); all the past sins of the person whose saying (of Amin) coincides with that of the angels, will be forgiven".

Sahih al-Bukhari Book 10, Hadith 177