Sunan Ibn Majah — Establishing the Prayer and the Sunnah Regarding Them — Hadith No: 876
Translation
It was narrated from Anas bin Malik that the Messenger of Allah (ﷺ) said:“When the Imam says: ‘Sami’ Allahu liman hamidah (Allah hears those who praise Him),’ say: ‘Rabbana wa lakal-hamd (O our Lord, to You is the praise).’”