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).’”