The Messenger of Allah (ﷺ) said: When anyone amongst you utters Amin and the angels In the heaven also utter Amin and (the Amin) of the one synchronises with (that of) the other, all his previous sins are pardoned.
Reference: Sahih Muslim 410 d In-book : Book 4, Hadith 80 USC-MSA web (English) : Book 4, Hadith 814 (deprecated)
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