Abu Hurairah Radiyallahu 'anhu narrates that Rasullullah Sallallahu 'alaihi wasallam said: Say Amin, when the Imam has recited ''Ghairil maghdiubi 'alaihim waladdallin". For undoubtedly whosever's words (Amin) coincides with the words of the angels, all his past sins are forgiven. (Bukhari: 782)