The Book of the Commencement of the Prayer
11/33. Chapter:
Imam saying Amin out loud
Sunan an-Nasai
أَخْبَرَنَا عَمْرُو بْنُ عُثْمَانَ، قَالَ حَدَّثَنَا بَقِيَّةُ، عَنِ الزُّبَيْدِيِّ، قَالَ أَخْبَرَنِي الزُّهْرِيُّ، عَنْ أَبِي سَلَمَةَ، عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا أَمَّنَ الْقَارِئُ فَأَمِّنُوا فَإِنَّ الْمَلاَئِكَةَ تُؤَمِّنُ فَمَنْ وَافَقَ تَأْمِينُهُ تَأْمِينَ الْمَلاَئِكَةِ غَفَرَ اللَّهُ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ " .
It was narrated that Abu Hurairah said: "The Messenger of Allah (ﷺ) said: 'When the reciter says Amin, then say: "Amin" too, for the angels say Amin and if a person's Amin coincides with the Amin of the angels, Allah will forgive his previous sins"
Reference : Sunan an-Nasa'i 925In-book reference : Book 11, Hadith 50English translation : Vol. 2, Book 11, Hadith 926