Numbers 35:26-28

26But if the manslayer at any time goes beyond the border of his city of refuge to which he may flee, 27and the blood avenger finds him outside the border of his city of refuge, and the blood avenger kills the manslayer, he will not be guilty of blood 28because he should have remained in his city of refuge until the death of the high priest. But after the death of the high priest the manslayer shall return to the land of his possession.