The Messenger of Allah ﷺ said, "When the imam says 'Ameen,' say 'Ameen' as well, for whoever's 'Ameen' coincides with that of the angels will have his past sins forgiven."
أخبرنا قتيبة عن مالك عن ابن شهاب عن سعيد وأبي سلمة أنهما أخبراه
أن رسول الله ﷺ قال إذا أمن الإمام فأمنوا فإنه من وافق تأمينه تأمين الملائكة غفر له ما تقدم من ذنبه
Bukhari transmitted it in the Book of Adhan, chapter on the imam saying the Ameen aloud, hadith no. 780, and Muslim in the Book of Prayer, chapter on saying "sami Allahu liman hamidah," "Rabbana wa laka al-hamd," and the Ameen, hadith no. 410, from the hadith of Malik through this same chain. It is also found in Muwatta (Yahya) 1/87, and in Kubra, hadith no. 1000.