expression
teaching your grandmother to suck eggs; preaching to the choir
Idiomatic expression meaning to give unnecessary instruction to someone who is already an expert. Literally 'preaching to the Buddha'.
彼にパソコンの使い方を教えるなんて、釈迦に説法だよ。
Teaching him how to use a computer is like teaching your grandmother to suck eggs.
From the Buddhist concept that it is pointless to preach to the Buddha, who is already enlightened. The exact historical derivation is uncertain.