TSK · Numbers 35:19

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

then the congregation shall judge between the smiter and the avenger of blood according to these ordinances;

lest the avenger of blood pursue the manslayer, while his heart is hot, and overtake him, because the way is long, and smite him mortally; whereas he was not worthy of death, inasmuch as he hated him not in time past.

that the manslayer that killeth any person unwittingly [and] unawares may flee thither: and they shall be unto you for a refuge from the avenger of blood.

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

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