question #1 Sahih Muslim 410c, Book 4, Hadith 79 : What is the significance of saying ‘Amin’ during prayer in Islam?

The Messenger of Allah (ﷺ) said: When anyone amongst you utters Amin in prayer and the angels in the sky also utter Amin, and this (utterance of the one) synchronises with (that of) the other, all his previous sins are pardoned.