1 Samuel 28:1-2

1And it came to pass in those days, that the Philistines gathered their hosts together for warfare, to fight with Israel. And Achish said unto David, Know thou assuredly, that thou shalt go out with me in the host, thou and thy men. 2And David said to Achish, Therefore thou shalt know what thy servant will do. And Achish said to David, Therefore will I make thee keeper of my head for ever.