The Messenger of Allah (ﷺ) said: "When the Imam says: 'Sami Allahu liman hamidah (Allah hears the one who praises Him),' then say: 'Rabbana wa lakal-hamd,' (Our Lord, and to You be the praise).' Whoever says that and it coincides with the angels saying it, his previous sins will be forgiven."
Reference: Sunan an-Nasa'i 1063 In-book : Book 12, Hadith 35 English translation : Vol. 2, Book 12, Hadith 1064
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