Maliks Muwatta Book 003, Hadith Number 048.

Section : Saying 'Amin' behind the Imam.

Yahya related to me from Malik from Sumayy, the mawla of Abu Bakr, from Abu Salih as Samman, from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "When the imam has said, 'not of those with whom You are angry nor of those who are in error,' say 'Amin', for the previous wrong actions of the one whose utterance coincides with that of the angels, are forgiven him."