Is there any Hadith about saying 'AAMEEN' loudly in the Fard prayer after Sura Fatiha?
Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
About saying Ameen, here is the Hadith:
It is reported by Imams Al Bukhari and Muslim from Abu Huraira that the Prophet said: "When the Imam says: 'Ghairi-il Maghdubi 'Alaihim Walad-Dalin' (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 of Ameen coincides with that of the angels, then his past sins will be forgiven".
Allah knows best.
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