釈迦に説法
1
teaching your grandmother to suck eggs;teaching something to someone who knows more than you;preaching to the choir;lecturing to the Buddha
釈迦に説法とは存じますが、医者も禁煙されたほうがよろしいのではないでしょうか。
I'm sure you don't need me to tell you this, but wouldn't it be a good idea to stop smoking?