The Messenger of Allah ﷺ said, "When the imam says, 'Not of those who have earned Your anger, nor of those who go astray' [Fatihah 1:7], say 'Ameen,' for whoever's utterance coincides with that of the angels will have his past sins forgiven."
أخبرنا قتيبة عن مالك عن سمي عن أبي صالح
أن رسول الله 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 narrated it, Adhan, chapter on the imam's follower saying "amin" aloud, no. 782, and Muslim, no. 409 (see also the previous hadith from the hadith of Malik with the same wording, and it is in Muwatta [Yahya], 1/87, and Kubra, no. 1001).