Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 831
Abu Huraira reported Allah's Messenger (may peace be upon him) saying: The Imamis 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 Lordfor Thee be the praise." And when he prays, standing, you should pray standing. And when he prays sitting, all of you should pray sitting.