The Book of Prayers (Kitab Al-Salat)

Muslim :: Book 4 : Hadith 814

Abu Harare reported: The Messenger of Allah (may peace be upon him) 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.

Source materials are from the University of Southern California MSA site
Hadith eBooks converted from Imaan Star