sutor, ne ultra crepidam

Cobbler, no further than the sandal!

Notes

Thus, don't offer your opinion on things that are outside your competence. It is said that the Greek painter Apelles once asked the advice of a cobbler on how to render the sandals of a soldier he was painting. When the cobbler started offering advice on other parts of the painting, Apelles rebuked him with this phrase in Greek, and it subsequently became a popular Latin expression.