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."
Muwatta Malik Book 3, Hadith 48