2 Samuel 15:12, 31

And Absalom sent for Ahithophel the Gilonite, David's counsellor, from his city, even from Giloh, while he offered sacrifices. And the conspiracy was strong; for the people increased continually with Absalom.

And one told David, saying, Ahithophel is among the conspirators with Absalom. And David said, O LORD, I pray thee, turn the counsel of Ahithophel into foolishness.