Musnad Ahmad 30:2524

Yahya narrated to us, from Muhammad, Abu Salamah narrated to us, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "When the Imam says the Takbir, then say the Takbir. When he bows, then bow. When he prostrates, then prostrate. And when he prays sitting, then pray sitting."