Sunan Ibn Majah
852
Sahih+3

It was narrated that Abu Hurairah said:The Messenger of Allah (ﷺ) said: “When the reciter says Amin, then say Amin, for 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