Yahya related to me from Malik from Abu'z Zinad from al-Araj fromAbu Hurayra that the Messenger of Allah, may Allah bless him and granthim peace, said, "When one of you says 'Amin' and the angels in thesky say 'Amin' so that one coincides with the other, his previouswrong actions are forgiven him."
App reference: Book 3, Hadith 49
USC-MSA web (English) reference: Book 3, Hadith 49
Arabic reference: Book 3, Hadith 196