1 Samuel 21:15

13He changed his behavior before them and pretended to be insane in their hands, and scribbled on the doors of the gate, and let his spittle fall down on his beard.

14Then Achish said to his servants, “Look, you see the man is insane. Why then have you brought him to me?

15Do I lack madmen, that you have brought this fellow to play the madman in my presence? Should this fellow come into my house?”