Yahya related to me from Malik from Ibn Shihab that Sa'id ibn al-Musayyab and Abu Salama ibn Abd ar-Rahman told him from Abu Hurayrathat the Messenger of Allah, may Allah bless him and grant him peace,said, "When the imam says 'Amin', say 'Amin', for the one whose 'Amin'coincides with the 'Amin' of the angels - his previous wrong actionsare forgiven him."Ibn Shihab said, "The Messenger of Allah,may Allah bless him and grant him peace, used to say 'Aameen'(extending it)."
App reference: Book 3, Hadith 47
USC-MSA web (English) reference: Book 3, Hadith 47
Arabic reference: Book 3, Hadith 194