Abu Hurairah Radiyallahu 'anhu narrates that Rasullullah Sallallahu 'alaihi wasallam said: When anyone of you says Amin (at the end of reciting Surah Fatihah), the angels at the same time also say Amin in the skies. If the former Amin coincides with the latter, then all his past sins are forgiven. (Bukhari: 781)