Musannaf Ibn Abi Shayba 3:221

Abu Bakr narrated to us, he said: Muhammad ibn Fudayl narrated to us, from Mutarrif, from Amir, who said: The people behind the Imam should not say, "Allah hears those who praise Him" (Sami' Allahu liman hamidah). Rather, they should say, "O Allah, our Lord, to You is the praise" (Allahumma Rabbana laka al-hamd).