Sunan an-Nasa'i 928
سنن النسائي#928SahihIt was narrated from Abu Hurairah that:The Messenger of Allah (ﷺ) said: "When the Imam says Amin, say Amin, for if a person's Amin coincides with the Amin of the angels, his previous sins will be forgiven
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 - Agreed Upon
In-book reference: 11:53