Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 828
Abu Huraira reported: The Messenger of Allah (may peace be upon him) while teaching us (the principles of faith), said: Do not try to go ahead of the Imam, recite takbir when he recites it. and when he says:" Nor of those who err," you should say Amin, bow down when lie bows down, and when he says:" Allah listens to him who praises Him," say:" O Allah, our Lord, to Thee be the praise".