A word from the mouth is like a stone from a sling.

If you keep your mouth shut, the flies won't get in

So David prevailed over the Philistine with a sling and with a stone, and smote the Philistine, and slew him; but there was no sword in the hand of David.

You can't get blood out of a stone

Word of mouth is everything.

Word of mouth is the best medium of all.