Silence is golden / If you keep your mouth shut, you won't put your foot in it
Literally 'Flies don't enter a closed mouth'. Advises keeping quiet to avoid trouble or saying something you'll regret.
Better say nothing in the meeting. Silence is golden.
—Should I tell him what I know? —If you keep your mouth shut, you won't put your foot in it.
He learned the hard way that silence is golden.