Say 'Rabbana wa lakal-hamd' when you hear an imam say 'Sami Allahu liman hamidah'.

This hadith speaks about the power of praising Allah and how it can lead to forgiveness from Him. It was narrated by Abu Hurairah that the Prophet Muhammad said that when an Imam says “Sami Allahu liman hamidah” which means “Allah hears those who praise Him” then one should say “Rabbana wa lakal-hamd” which means “Our Lord and to You be all praise”. If this coincides with what the angels are saying then all previous sins will be forgiven. This hadith teaches us that we should always remember to thank and praise God for His blessings in our lives as this is a way for us to receive His mercy and forgiveness.