A proverb meaning you should go to a specialist for the best results; literally, 'for mochi, (go to) the mochi shop.'
パソコンの修理は自分でやるよりプロに頼んだほうがいい。餅は餅屋だよ。
It's better to have a professional fix your computer than to do it yourself. You should go to the specialist.
家具の塗装は難しいから、餅は餅屋と言って業者に任せた。
Painting the furniture was difficult, so I followed the proverb and left it to a contractor.
和菓子の注文は店に直接依頼した。餅は餅屋だからね。
I ordered Japanese sweets directly from the shop. After all, specialists know best.