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.”
Sunan Ibn Majah Book 5, Hadith 50
← Previous Hadith “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.”

Sunan Ibn Majah Book 5, Hadith 49 Next Hadith → “The people stopped saying Amin, but when the Messenger of Allah (ﷺ) said ‘Not (the way) of those who earned Your Anger, nor of those who went astray’ he would say Amin, until the people in the first row could hear it, and the mosque would shake with it.

Sunan Ibn Majah Book 5, Hadith 51