The Messenger of Allah ﷺ said, "When one of you says 'Ameen' and the angels in the heavens say 'Ameen,' and the two coincide with each other, his previous sins will be forgiven."
أخبرنا قتيبة عن مالك عن أبي الزناد عن الأعرج
أن رسول الله ﷺ قال إذا قال أحدكم آمين وقالت الملائكة في السماء آمين فوافقت إحداهما الأخرى غفر له ما تقدم من ذنبه
Bukhari reported it in the chapter on adhan, section on the merit of saying "Ameen," hadith no. 781, from the hadith of Malik through the same chain, and it is also found in Muwatta (Yahya version) 1/88, and in Kubra, hadith no. 1002. Muslim also reported it, hadith no. 410 (see the previous hadith), through another route from Abu Zinad with the same chain.