Sahih Al Bukhari – Book of Prophetic Commentary On The Qur'an (Tafseer Of The Prophet (pbuh))

Volumn 006, Book 060, Hadith Number 002.
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Narated By Abu Huraira RA: Allah's Apostle PBUH 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."