When Saul had watched David going out to fight the Philistine, he'd asked Abner the army commander, “Abner, whose son is that young man?” “On your life, Your Majesty, I do not know,” Abner replied.
As soon as David returned from killing the Philistine, Abner took him and brought him before Saul. David was still clutching the Philistine's head in his hand.