´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).’”
Hadith ReferenceSunan Ibn Majah / Establishing the Prayer and the Sunnah Regarding Them / Hadith: 876
Hadith Gradingالألبانی:صحيح لغيره | زبیر علی زئی:بخاري ومسلم