Musannaf Ibn Abi Shayba 3:220

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Salamah, from Abu al-Ahwas, from ‘Abdullah, who said: When the Imam says, "Sami’a Allahu liman hamidah" (Allah hears those who praise Him), those behind him should say, "Allahumma Rabbana laka al-hamd" (O Allah, our Lord, to You is the praise).