the.com/let sleeping dogs lie

means Don't bring up or interfere with an old problem that's currently quiet, because doing so will likely cause trouble.

from A proverb that's been around in English since at least the 14th century in various forms, with the modern wording popularized through 19th-century usage (including a nod from Dickens). The image is plain and practical: a sleeping dog is harmless, but poke or wake it and it may snap — so farmers and travelers alike learned to step around the snoozing hound rather than test its temper. The saying simply exported that bit of barnyard wisdom to arguments, grudges, and old scandals.