Musannaf Ibn Abi Shayba 3:4781

Abu Usama narrated to us, he said: Ibn Abi Aruba narrated to us, he said: Qatada narrated to us, from Yunus ibn Jubayr, from Hattan ibn Abdullah, who said: Abu Musa led us in prayer. When he turned away [from the prayer], he said: The Prophet of Allah, may Allah bless him and grant him peace, addressed us and explained to us our Sunnah and taught us our prayer. He said: "When the Imam says the Takbir, then say the Takbir, and when he bows, then bow, for the Imam bows before you and rises before you."