Musannaf Ibn Abi Shayba 3:4767

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Abu Ishaq, from Abu al-Ahwas, who said: Abdullah said: The Imam was only appointed to be followed. So when he says the Takbir, then say the Takbir, and when he bows, then bow. And when he prostrates, then prostrate, for he is the first to rise and the first to place [his forehead on the ground].