Maliks Muwatta Book 3, Hadith Number 47.

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 Hurayra that 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 actions are forgiven him.”

Ibn Shihab said, “The Messenger of Allah, may Allah bless him and grant him peace, used to say ‘Aameen’ (extending it).”