(18) باب التَّسْمِيعِ وَالتَّحْمِيدِ وَالتَّأْمِينِ
Abu Harare reported:
The Messenger of Allah (ﷺ) said: When anyone amongst you utters Amin and the angels In the heaven also utter Amin and (the Amin) of the one synchronises with (that of) the other, all his previous sins are pardoned.
Reference: Sahih Muslim 410 d
In-book reference: Book 4, Hadith 80
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 814