Sunan an-Nasa'i 925
سنن النسائي#925SahihIt was narrated that Abu Hurairah said:"The Messenger of Allah (ﷺ) said: 'When the reciter says Amin, then say: "Amin" too, for the angels say Amin and if a person's Amin coincides with the Amin of the angels, Allah will forgive his previous sins
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 11:50