Sunan Abi Dawud 936
سنن أبي داود#936SahihAbu Hurairah reported the Messenger of Allah (May peace be upon him) as saying; When the Imam says Amin, say Amin, for if anyone’s utterance of Amin synchronnises with that of the angles, he will be forgiven his past sins. Ibn shihab (al Zuhrl) said; The Messenger of Allah (May peace be upon him) used to say Amin (At the end of the Fatihah)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Al-AlbaniSahih
Muhammad Muhyi Al-Din Abdul HamidSahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Bukhari (780) Sahih Muslim (410)
In-book reference: 2:547