It wasnarrated from Abu Sa'eed Al-Khudri that he heard the MessengerofAllah (ﷺ) say:
"When the Imam says: 'Sami' Allahu limanhamidah(Allah hears those who praise Him),' say: 'Allahumma,Rabbana walakal-hamd (O Allah! O our Lord! To You is the praise).'" (ID)
English reference (ID): Vol. 1, Book 5, Hadith 877 (ID)
Arabic reference (ID): Book 5, Hadith 926 (ID)