2 Samuel 3:29 (KJV)

And when Abner was returned to Hebron, Joab took him aside in the gate to speak with him quietly, and smote him there under the fifth rib, that he died, for the blood of Asahel his brother.

And afterward when David heard it, he said, I and my kingdom are guiltless before the Lord for ever from the blood of Abner the son of Ner:

Let it rest on the head of Joab, and on all his father’s house; and let there not fail from the house of Joab one that hath an issue, or that is a leper, or that leaneth on a staff, or that falleth on the sword, or that lacketh bread.

So Joab and Abishai his brother slew Abner, because he had slain their brother Asahel at Gibeon in the battle.

And David said to Joab, and to all the people that were with him, Rend your clothes, and gird you with sackcloth, and mourn before Abner. And king David himself followed the bier.