The Book of the Commencement of the Prayer
11/33. Chapter:
Imam saying Amin out loud
Sunan an-Nasai
أَخْبَرَنَا إِسْمَاعِيلُ بْنُ مَسْعُودٍ، قَالَ حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، قَالَ حَدَّثَنِي مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا " قَالَ الإِمَامُ { غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلاَ الضَّالِّينَ } فَقُولُوا آمِينَ فَإِنَّ الْمَلاَئِكَةَ تَقُولُ آمِينَ وَإِنَّ الإِمَامَ يَقُولُ آمِينَ فَمَنْ وَافَقَ تَأْمِينُهُ تَأْمِينَ الْمَلاَئِكَةِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
It was narrated that Abu Hurairah said: "The Messenger of Allah (ﷺ) said: 'When the Imam says: Not (the way) of those who earned Your anger, nor of those who went astray, say: 'Amin' for the angels say Amin and the Imam says Amin, and if a person's Amin coincides with the Amin of the angels, his previous sins will be forgiven.'"
Reference : Sunan an-Nasa'i 927In-book reference : Book 11, Hadith 52English translation : Vol. 2, Book 11, Hadith 928