Musnad Ahmad 788:27

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Qatadah, from Yunus ibn Jubayr, from Hattan ibn 'Abd Allah al-Raqashi, from Abu Musa al-Ash'ari, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When the Imam says: 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say: 'Rabbana wa lakal-hamd' (Our Lord, and to You is the praise), for Allah the Almighty hears you. For indeed, Allah the Almighty has decreed upon the tongue of His Prophet (peace and blessings of Allah be upon him): 'Sami' Allahu liman hamidah'."