Cobbler, stay with your lasts.
The story is from Pliny the Elder. A shoemaker criticized the sandal in a painting by Apelles, and the painter corrected it. When the cobbler went on to criticize the leg, the painter told him to stop.
The German use of the phrase is not an instruction in modesty. It asserts the opposite of what it appears to say: within your last, your judgment outranks anyone’s.