Muwatta Malik 194
موطأ مالك#194SahihYahya 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Salim al-HilaliSahih
In-book reference: 3:27