1 Samuel 26:6-9

6Then David said to Ahimelech the Hittite, and to Joab's brother Abishai the son of Zeruiah, "Who will go down with me into the camp to Saul?" And Abishai said, "I will go down with you."

7So David and Abishai went to the army by night. And there lay Saul sleeping within the encampment, with his spear stuck in the ground at his head, and Abner and the army lay around him.

8Then Abishai said to David, "God has given your enemy into your hand this day. Now please let me pin him to the earth with one stroke of the spear, and I will not strike him twice."

9But David said to Abishai, "Do not destroy him, for who can put out his hand against the Lord's anointed and be guiltless?"