Sahih Muslim Book 4, Hadith 96
The Messenger of Allah (ﷺ) said: Verily the Imam is a shield, say prayer sitting when he says prayer sitting. And when he says:" Allah listens to him who praises Him," say:" O Allah, our Lord, to Thee be the praise." and when the utterance of the people of the earth synchronises with that of the beings of heaven (angels), all the previous sins would be pardoned.
" Nor of those who err, say Amin" and added:" And don't rise up ahead of him."
The Imam is appointed to be followed. So recite takbir when he recites it, and bow down when he bows down and when he utters:" Allah listens to him who praises Him," say" O Allah, our Lord, for Thee be the praise." And when he prays, standing, you should pray standing. And when he prays sitting, all of you should pray sitting.