Musannaf Ibn Abi Shayba 3:222

Abu Bakr narrated to us, he said: Yahya ibn Abi Bukayr narrated to us, he said: Zuhayr ibn Muhammad narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Sa'id ibn al-Musayyib, from Abu Sa'id al-Khudri, that he heard the Prophet, peace and blessings of Allah be upon him, saying: "When your Imam says, 'Allah hears those who praise Him' (Sami' Allahu liman hamidah), then say, 'O Allah, our Lord, to You is the praise' (Allahumma Rabbana lakal-hamd)."