Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "The imam has only been appointed to be followed. So when he bows, you bow; and when he raises his head, you raise yours; and do not go ahead of him in bowing, in raising the head, or in prostrating. And if your concern is to catch up with what he has done before you, then he prostrates before you and raises his head before you, so you will catch up with that afterwards."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9346