Musannaf Ibn Abi Shayba 3:216

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "When the Imam says, 'Allah hears those who praise Him' (Sami‘a Allahu liman hamidah), then say: 'O Allah, our Lord, to You is the praise' (Allahumma Rabbana laka al-hamd)."