Musnad Ahmad 32:709

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When the Imam says: 'Sami'a Allahu liman hamidah' (Allah hears those who praise Him), then say: 'Rabbana wa laka al-hamd' (Our Lord, and to You is the praise)."