Chatter not overmuch so that thou speak out every word that come to thy mind; for men's eyes and ears are everywhere trained upon thy mouth. Beware lest it be thy undoing. More than all watchfujlness watch thy mouth and over what thou hearest harden thy heart.
For a word is a bird: once released no man can recapture it. First count the secrets of thy mouth: then bring out thy words by number. For the instruction of a mouth is stronger than the instruction of war.
Treat not lightly the word of a king: let it be healing for thy flesh.
From "the proverbs of the sage Ahiqar, a legendary counsellor at the court of the Assyrian kings Sennacherib and Esarhaddon in the early seventh century BC," as quoted by Nicholas Ostler in Empires of the World at 83-4 (2005). I still can't quite figure out that last line.