1 Samuel 28:1-2

1And it came to pass in those days that the Philistines gathered together their armies for warfare to fight against Israel. And Achish said to David, Know thou assuredly that thou shalt go out with me to the camp, thou and thy men.

2And David said to Achish, Thereby thou shalt know what thy servant can do. And Achish said to David, Therefore will I make thee keeper of my person for ever.