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?