2 Samuel 3:26-27

26Then Joab left David. He sent men to run after Abner with a message. They found him at the well at Sirah and they brought him back to Hebron. But David did not know that.

27When Abner returned to Hebron, Joab met him at the city's gate. Then Joab took him into a corner, as if he wanted to speak with Abner alone. But he pushed his knife into Abner's stomach. Joab killed Abner because Abner had killed Asahel, Joab's brother.