the.com/practice what you preach

means Your actions should match the standards and advice you hold others to — don't just talk the talk.

from The phrase grows from a much older contrast, found across centuries of moral and religious writing, between preaching and doing — Chaucer's Parson, for instance, was praised for teaching Christ's lore and following it himself. 'Preach' comes from Latin praedicare, 'to proclaim publicly'; the pairing with 'practice' hardened into its current fixed idiom sometime in the early modern period, used to call out hypocrites who demand of others what they won't demand of themselves.