TSK · Numbers 35:19

And they shall be unto you cities for refuge from the avenger; that the manslayer die not, until he stand before the congregation in judgment.

Then the congregation shall judge between the slayer and the revenger of blood according to these judgments:

Lest the avenger of the blood pursue the slayer, while his heart is hot, and overtake him, because the way is long, and slay him; whereas he [was] not worthy of death, inasmuch as he hated him not in time past.

That the slayer that killeth [any] person unawares [and] unwittingly may flee thither: and they shall be your refuge from the avenger of blood.

And he said, They [were] my brethren, [even] the sons of my mother: [as] the LORD liveth, if ye had saved them alive, I would not slay you.

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