Musannaf Ibn Abi Shayba 3:219

Abu Bakr narrated to us, he said: Abu Khalid narrated to us, from Ibn ‘Ajlan, from Zayd ibn Aslam, from Abu Salih, from Abu Hurayrah, who said: The Prophet, may Allah bless him and grant him peace, said: "The Imam was only appointed to be followed. So when he bows, then bow; and when he says, 'Allah hears those who praise Him' (Sami‘ Allahu liman hamidah), then say, 'O Allah, our Lord, to You is the praise' (Allahumma Rabbana laka al-hamd)."