The Prophet ﷺ said, "When the imam says, 'not of those who have incurred wrath, nor of those who have gone astray' [Fatihah 1:7], say 'Ameen,' for whoever's utterance coincides with that of the angels will have his past sins forgiven."
حدثنا القعنبي عن مالك عن سمي مولى أبي بكر عن أبي صالح السمان
أن النبي ﷺ قال إذا قال الإمام {غير المغضوب عليهم ولا الضالين} فقولوا آمين فإنه من وافق قوله قول الملائكة غفر له ما تقدم من ذنبه
Bukhari reported it in the Adhan, chapter on the follower saying "Amin" aloud, no. 782, from Abdullah bin Maslama Qanabi. Muslim also reported it, in Prayer, chapter on saying "Allah hears whoever praises Him," saying "Our Lord, to You is due all praise," and saying "Amin," no. 409, from the hadith of Malik through him. It is also found in Muwatta (Yahya narration), 12/87, and (Qanabi narration, p. 141).