As Saul watched David1 going out to meet the Philistine, he said to Abner, commander of the army, "Abner,2 whose son is that young man?" Abner replied, "As surely as you live, O king, I don't know."
References for 1 Samuel 17:55
The king said, "Find out whose son this young man is."
As soon as David returned from killing the Philistine, Abner took him and brought him before Saul, with David still holding the Philistine's head.
"Whose son are you, young man?" Saul asked him. David said, "I am the son of your servant Jesse3 of Bethlehem."
References for 1 Samuel 17:58