stick to the knitting
1. To mind one's own business. Oh, stick to the knitting—I don't need to hear your opinion of every little thing I do.
2. To do only what one knows or is familiar with. You can't just stick to the knitting—you have to try new things too.
stick to the (or your) knitting
(of an organization) concentrate on a known core area of business activity rather than diversify into other areas in which it has no experience. informal