صحيح مسلم

Abu Huraira reported Allah's Messenger (ﷺ) saying: 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.