The Messenger of Allah ﷺ said: When anyone amongst you utters Amin in prayer and the angels in the sky also utter Amin, and this (utterance of the one) synchronises with (that of) the other, all his previous sins are pardoned. (Using translation from Muslim 410c)
وَقَالَ رَسُولُ اللهِ ﷺ إِذَا قَالَ أَحَدُكُمْ آمِينَ وَالْمَلَائِكَةُ فِي السَّمَاءِ فَيُوَافِقُ إِحْدَاهُمَا الْأُخْرَى غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ