Allah's Messenger (ﷺ) said, "When the Imam, during the prayer, says, "Allah hears him who praises Him', say: 'O Allah! Our Lord! All the praises are for You/, for if the saying of anyone of you coincides with the saying of the angels, his past sins will be forgiven."
Sahih al-Bukhari Book 59, Hadith 39
← Previous Hadith Once Gabriel promised the Prophet (that he would visit him, but Gabriel did not come) and later on he said, "We, angels, do not enter a house which contains a picture or a dog."

Sahih al-Bukhari Book 59, Hadith 38 Next Hadith → The Prophet (ﷺ) said, "As long as any-one of you is waiting for the prayer, he is considered to be praying actually, and the angels say, 'O Allah! Be merciful to him and forgive him', (and go on saying so) unless he leaves his place of praying or passes wind (i.e. breaks his ablution).

Sahih al-Bukhari Book 59, Hadith 40