1 Kings 2:5-6

5“Moreover you know also what Joab the son of Zeruiah did to me, and what he did to the two commanders of the armies of Israel, to Abner the son of Ner and Amasa the son of Jether, whom he killed. And he shed the blood of war in peacetime, and put the blood of war on his belt that was around his waist, and on his sandals that were on his feet.

6Therefore do according to your wisdom, and do not let his gray hair go down to the grave in peace.