Do not answer a fool according to his folly, or you will be like him yourself.

Related:

• Answer a fool according to his folly. -- Old Testament -
  Proverbs xxvi,...
• Answer a fool according to his folly, lest he be wise in his own conceit.
  Proverbs, 26...
• A fool finds no pleasure in understanding but delights in airing his own opinions.
  Miscellaneous...
• A fool gives full vent to his anger,
  but a wise man keeps himself under control Miscellaneous...
• A fool shows his annoyance at once, but a prudent man overlooks an insult.
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• As a dog returns to his vomit, so a fool returneth to his folly.
  Seest thou a man wise in his own conceit? There is...
• A fool finds pleasure in evil conduct, but a man of understanding delights in wisdom.
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• If A fool persists in his folly he shall become wise.
  William...
• Even a fool is thought wise if he keeps silent, and discerning if he holds his tongue.
  Miscellaneous...