釈迦 しゃか説法 せっぽう
shakani seppō

Idiomatic Expression

1. teaching your grandmother to suck eggs; teaching something to someone who knows more than you; preaching to the choir; lecturing to the Buddha — literal

釈迦 しゃか説法 せっぽうとは ぞんじますが、医者 いしゃ禁煙 きんえんされたほうがよろしいのではないでしょうか。
shakani seppōtowa zonjimasuga ishamo kin'en sareta hōga yoroshii nodewanaideshōka.
I'm sure you don't need me to tell you this, but wouldn't it be a good idea to stop smoking?