Musannaf Ibn Abi Shayba 3:1422

Abu Khalid al-Ahmar narrated to us, from Ibn ‘Ajlan, from Zayd ibn Aslam, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The Imam was only appointed to be followed. So when he says the takbir, say the takbir, and when he recites, remain silent."