It is narrated from Hazrat Abu Musa (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: When the Imam says "Sami' Allahu liman hamidah," you should say "Rabbana lakal hamd." Allah will listen to you, because Allah has decreed through the tongue of His Prophet (peace and blessings be upon him) that whoever praises Allah, Allah listens to him.