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).’”

حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ أَنَسِ بْنِ مَالِكٍ، . أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ " إِذَا قَالَ الإِمَامُ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ . فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ " .