Musannaf Ibn Abi Shaybah

Hazrat Hattan bin Abdullah narrates that Hazrat Abu Musa led us in prayer. When he finished the prayer, he said that the Prophet (peace and blessings be upon him) delivered a sermon to us, in which he explained the religion to us and taught us how to pray. In it, he said: When the Imam says the takbeer, you should also say the takbeer. When the Imam bows (for ruku), you should also bow. Because the Imam bows before you and raises his head before you.