Sahih Muslim 917
صحيح مسلم#917Sahihحَدَّثَنِي حَرْمَلَةُ بْنُ يَحْيَى، حَدَّثَنِي ابْنُ وَهْبٍ، أَخْبَرَنِي عَمْرٌو، أَنَّ أَبَا يُونُسَ، حَدَّثَهُ عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِذَا قَالَ أَحَدُكُمْ فِي الصَّلاَةِ آمِينَ . وَالْمَلاَئِكَةُ فِي السَّمَاءِ آمِينَ . فَوَافَقَ إِحْدَاهُمَا الأُخْرَى غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
Abu Huraira reported: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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 4:79