1 Samuel 26:15
And David said to Abner, Art not thou a valiant man? and who is like to thee in Israel? wherefore then hast thou not kept thy lord the king? for there came one of the people in to destroy the king thy lord. And David said to Abner, Art not thou a valiant man? and who is like to thee in Israel? wherefore then hast thou not kept thy lord the king? for there came one of the people in to destroy the king thy lord.
This took place after Saul had slain the priests of the Lord, when Abiathar son of Ahimelech son of Ahitub escaped and fled to David for refuge. David received him with a promise of safeguard, since Saul sought both their lives, while David himself was still hunted with no place of rest or security. The words were spoken after David had reached a safe distance from Saul's camp, when he stood on the top of a hill and called out with a loud voice to the people and to Abner.
He rebuked Abner for failing to guard Saul his king when one of the people had come into the camp to destroy him, appealing to Abner's standing as a brave man with none like him in Israel to press his duty to protect the Lord's anointed. The rebuke was proof of David's second sparing of his sovereign's life, showing respect for the Lord's anointed rather than harming him, and it was backed by the taken spear and cruse of water as evidence that David could have destroyed Saul but did not. That exposure of failure stung Abner and rankled in him, feeding his hatred of David and his resolve to advance his own ambitions by dividing Israel.