1 Samuel 26:11
The LORD forbid that I should stretch forth mine hand against the LORD’s anointed: but, I pray thee, take thou now the spear that is at his bolster, and the cruse of water, and let us go. The LORD forbid that I should stretch forth mine hand against the LORD’s anointed: but, I pray thee, take thou now the spear that is at his bolster, and the cruse of water, and let us go.
This took place after Saul had slain the Lord's priests, when Abiathar the son of Ahimelech escaped and fled to David, who received him with the promise that whoever sought David's life sought his life also but with David he would be in safeguard, while Saul still hunted David. David had served Saul obediently, and his humble, irreproachable, faithful conduct rebuked Saul's rebellious course, while Saul made the slaying of David the great object of his life. David often had Saul in his power and was urged by the men he commanded to slay him, yet he refused because God had anointed Saul, even though David knew he himself was chosen of God to rule Israel, thus respecting God's choice of ruler.
On this occasion Saul lay asleep in his night camp with his spear stuck in the ground and a cruse of water at his head, surrounded by Abner and soldiers locked in slumber. David had entered the camp with Abishai under cover of darkness, and when Abishai asked permission to strike Saul dead at once with the spear, David forbade him to stretch out his hand against the Lord's anointed and remain guiltless. In refusing he would not take vengeance into his own hands but left Saul's judgment to the Lord, who would smite him, or let his day come to die, or let him perish in battle. Instead of killing Saul he ordered the spear and the cruse of water to be taken as proof and withdrew, and they escaped unseen and unheard because a deep sleep from the Lord had fallen on the whole camp, showing how easily the Lord can weaken the strongest, remove prudence from the wisest, and baffle the skill of the most watchful. Rather than strike Saul, David chose to find refuge among the Philistines and by his prudent, humble course made even his enemies to be at peace with him, remaining there until Saul's death.