Sunan Ibn Majah
851
Sahih+3

It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:“When the reciter says Amin, then say Amin, for the angels say Amin, and if a person’s Amin coincides with the Amin of the angels, his previous sins will be forgiven.”

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi