Even a fool is thought wise if he keeps silent, and discerning if he holds his tongue.

Related:

• Even a fool who keeps silent is considered wise; when he closes his lips,
  he is deemed intelligent. -- Proverbs 17...
• A fool gives full vent to his anger,
  but a wise man keeps himself under control Miscellaneous...
• Even a fool, when he holdeth his peace, is counted wise.
  Old Testament -- Proverbs xvii,...
• Let a fool hold his tongue and he will pass for
  a sage...
• Let a fool hold his tongue and he will pass for a sage.
  Publilius...
• Let a fool hold his tongue and he will pass for a sage.
  Pubilius...