It was narrated from Abu Sa’eed Al-Khudri that he heard the Messenger of Allah (ﷺ) say:“When the Imam says: ‘Sami’ Allahu liman hamidah (Allah hears those who praise Him),’ say: ‘Allahumma, Rabbana wa lakal-hamd (O Allah! O our Lord! To You is the praise).’”

English translation · Sunan Ibn Majah