The Messenger of Allah ﷺ said, "When the imam says 'Ameen,' say 'Ameen' as well, for whoever's saying of 'Ameen' coincides with that of the angels will have his past sins forgiven."Ibn Shihab said that the Messenger of Allah ﷺ himself used to say "Ameen."
حدثنا القعنبي عن مالك عن ابن شهاب عن سعيد بن المسيب وأبي سلمة بن عبد الرحمن أنهما أخبراه
أن رسول الله ﷺ قال إذا أمن الإمام فأمنوا فإنه من وافق تأمينه تأمين الملائكة غفر له ما تقدم من ذنبه قال ابن شهاب وكان رسول الله ﷺ يقول آمين
Bukhari narrated it, Adhan, chapter on the imam saying the amin aloud, no. 780, and Muslim, Prayer, chapter on saying "sami Allahu li man hamidah," "hamdan kathiran," and the amin, no. 410, from the hadith of Malik through this same chain, and it is found in Muwatta (Yahya, 1/87) (and Qanabi, pp. 140, 141).