The Prophet (ﷺ) said, "Say Amin" when the Imam says it and if the Amin of any one of you coincides with that of the angels then all his past sins will be forgiven." Ibn Shihab said, "Allah's Messenger (ﷺ) used to Say "Amin."
Sahih al-Bukhari Book 10, Hadith 175
← Previous Hadith My father said, "The Prophet (ﷺ) used to prolong the first rak`a of the Zuhr prayer and shorten the second one and used to do the same in the Fajr prayer."

Sahih al-Bukhari Book 10, Hadith 174 Next Hadith → Allah's Messenger (ﷺ) said, "If any one of you says, "Amin" and the angels in the heavens say "Amin" and the former coincides with the latter, all his past sins will be forgiven."

Sahih al-Bukhari Book 10, Hadith 176