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.
أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا قَالَ أَحَدُكُمْ فِي الصَّلاَةِ آمِينَ وَالْمَلاَئِكَةُ فِي السَّمَاءِ آمِينَ فَوَافَقَ إِحْدَاهُمَا الأُخْرَى غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ