Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 826
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: The Imam is appointed, so that he should be followed, so don't be at variance with him. Recite takbir when he recites it; bow down when he bows down and when he says:" Allah listens to him who praises Him," say:" O Allah, our Lord, to Thee be the Praise." And when he (the Imam) prostrates, you should also prostrate, and when he says prayer sitting, you should all observe prayer sitting.