Musannaf Ibn Abi Shayba 3:217

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: 'Umar ibn Abi Salamah informed us, from his father, from Abu Hurayrah, who attributed it [to the Prophet (ﷺ)]: "When the Imam says: 'Sami'a Allahu liman hamidah' (Allah hears those who praise Him), then say: 'Allahumma Rabbana laka al-hamd' (O Allah, our Lord, to You is the praise)."