(18) باب التَّسْمِيعِ وَالتَّحْمِيدِ وَالتَّأْمِينِ
Abu Huraira reported:
The Messenger of Allah (ﷺ) said: When the reciter (Imam) utters:" Not of those on whom (is Thine) wrath and not the erring ones," and (the person) behind him utters Amin and his utterance synchronises with that of the dwellers of heavens, all his previous sins would be pardoned.
Reference: Sahih Muslim 410 f
In-book reference: Book 4, Hadith 82
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 816