Maliks Muwatta Book 003, Hadith Number 050.

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 says, 'Allah hears whoever praises Him,' say 'O Allah! Our Lord, praise be to You, 'for the previous wrong actions done by the one whose utterance coincides with that of the angels are forgiven."