The Messenger of Allah ﷺ said, "When the reciter says "not of those who have earned anger, nor of those who go astray" [Fatihah 1:7], and those behind him say 'Ameen,' and their saying coincides with the saying of the inhabitants of heaven 'Ameen,' his previous sins will be forgiven."
حدثنا يزيد قال أخبرنا محمد عن أبي سلمة
قال رسول الله ﷺ إذا قال القارئ {غير المغضوب عليهم ولا الضالين} فقال من خلفه آمين فوافق ذلك قول أهل السماء آمين غفر له ما تقدم من ذنبه
Its isnad is sahih. Muslim narrated it in the Book of Prayer, chapter on Tasbih and Tahmid, 1/307 no. 410, and Darimi in 1/314 no. 1246, in the Book of Prayer, chapter on the merit of saying Ameen, both with the same wording, and the rest narrated it with similar wordings.