The Prophet ﷺ said, "When the reciter says 'Ameen,' say 'Ameen' along with him, for the angels also say 'Ameen.' Whoever's 'Ameen' coincides with that of the angels will have his past sins forgiven."
أخبرنا محمد بن منصور قال حدثنا سفيان عن الزهري عن سعيد بن المسيب
عن النبي ﷺ قال إذا أمن القارئ فأمنوا فإن الملائكة تؤمن فمن وافق تأمينه تأمين الملائكة غفر له ما تقدم من ذنبه
Bukhari collected it in the Book of Supplications, Chapter on Saying Amin, hadith no. 6402, from the hadith of Sufyan bin Uyayna with the same chain, and it is also in Kubra, hadith no. 998.