1 Samuel 21:12-15

12Now David took these words to heart, and was very much afraid of Achish the king of Gath.

13So he changed his behavior before them, pretended madness in their hands, scratched on the doors of the gate, and let his saliva fall down on his beard.

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

15Have I need of madmen, that you have brought this fellow to play the madman in my presence? Shall this fellow come into my house?”