Narrated Abu Huraira:

Allah's Messenger (ﷺ) said, "When the Imam says: 'Ghair-il-Maghdubi `alaihim Walad-Dallin (i.e. not the path of those who earn Your Anger, nor the path of those who went astray (1.7)), then you must say, 'Ameen', for if one's utterance of 'Ameen' coincides with that of the angels, then his past sins will be forgiven."

Narrated Talha bin `Abdullah bin `Auf:

I offered the funeral prayer behind Ibn `Abbas and he recited Al-Fatiha and said, "You should know that it (i.e. recitation of Al-Fatiha) is the tradition of the Prophet (ﷺ) Muhammad.

Narrated Abu Huraira:

The Prophet (ﷺ) said, "When the Imam says 'Amin', then you should all say 'Amin', for the angels say 'Amin' at that time, and he whose 'Amin' coincides with the 'Amin' of the angels, all his past sins will be forgiven."

Narrated Abu Huraira:

The Prophet (ﷺ) said, "say ameen" when the imam says it and if the ameen of any one of you coincides with that of the angels then all his past sins will be forgiven." Ibn Shihab said, "Allah's Messenger (ﷺ) used to say "ameen."