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. (Using translation from Abū Dāʾūd 935)
"إذا قَال الإِمامُ: {غَيرِ الْمَغْضُوبِ عَلَيهِمْ وَلَا الضَّالِّينَ} فقال مَنْ خلفه (آمين) فوافقَ تأمينُهم تأمينَ الملائكةِ غفرَ (الله) له ما تقدَّمَ مِنْ ذنبه وما تأخر".