Sunan an-Nasa'i, Book 11, Hadith 55 (In-Book) ⓘ
It was narrated from Abu Hurairah that: The Messenger of Allah said: "If any one of you says: 'Amin' and the angels in Heaven say Amin, and the one coincides with the other, his previous sins will be forgiven."
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا قَالَ أَحَدُكُمْ آمِينَ وَقَالَتِ الْمَلاَئِكَةُ فِي السَّمَاءِ آمِينَ فَوَافَقَتْ إِحْدَاهُمَا الأُخْرَى غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
also referenced under: Hadith 930 (Reference) & Volume 2, Book 11, Hadith 931 (DarusSalam)