Characteristics of Prayer
Bukhari :: Book 1 :: Volume 12 :: Hadith 762
Narrated Abu Huraira:
Allah's Apostle said, "When the Imam says, "Sami' a-l-lahu Liman hamida," you should say, "Allahumma Rabbana laka-l-hamd." And if the saying of any one of you coincides with that of the angels, all his past sins will be forgiven."
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