The Messenger of Allah ﷺ said, "When the imam says 'Ameen,' say 'Ameen,' for whoever's 'Ameen' coincides with that of the angels will have his past sins forgiven."
أن رسول الله ﷺ قال: إذا أمن الإمام فأمنوا، فإنه من وافق تأمينه تأمين الملائكة غفر له ما تقدم من ذنبه.
Malik narrated it in "Prayer" (45) from Ibn Shihab, from Saeed bin Musayab and Abu Salama bin Abdul-Rahman, who both reported to him from Abu Hurairah, and he recounted the hadith. Bukhari narrated it in "Adhan" (780) from Abdullah bin Yusuf, and Muslim in "Prayer" (410) from Yahya bin Yahya, both of them from Malik, and he recounted it.