(174) باب التَّأْمِينِ وَرَاءَ الإِمَامِ
Abu Hurairah reported the prophet (ﷺ) as saying; when the imam recites " not of those with whom thou art angry, nor of those who go astray" (surah al-fatihah,verse 7) say Amin, for if one's words (utterance of amin) synchronise with those of the angles, he will be forgiven his past sins.
Reference: Sunan Abi Dawud 935
In-book reference: Book 2, Hadith 546
English translation: Book 2, Hadith 935