Sahih Muslim Book 4, Hadith 74
The Messenger of Allah (ﷺ) said: He who blesses me once, Allah would bless him ten times.
They (the Companions of the Holy Prophet) said: Apostle of Allah, how should we bless you? He (the Holy Prophet) observed: Say:" O Allah! bless Muhammad, his wives and his offspring as Thou didst bless Ibrahim, and grant favours to Muhammad, and his wives and his offspring as Thou didst grant favours to the family of Ibrahim; Thou art Praiseworthy and Glorious."
The Messenger of Allah (ﷺ) said: When the Imam says:" Allah listens to him who praises Him." you should say:" O Allah, our Lord for Thee is the praise." for if what anyone says synchronises with what the angels say, his past sins will be forgiven.