As soon as Joab had left David, he sent messengers after Abner, who brought him back from the well of Sirah; but David was unaware of it.

2 Samuel 3:25
Surely you realize that Abner son of Ner came to deceive you and to track your movements and all that you are doing."

2 Samuel 3:27
When Abner returned to Hebron, Joab pulled him aside into the gateway, as if to speak to him privately, and there Joab stabbed him in the stomach. So Abner died on account of the blood of Joab's brother Asahel.

And when Joab was come out from David, he sent messengers after Abner, which brought him again from the well of Sirah: but David knew it not.