Numbers 35:25-28

So the congregation shall deliver the manslayer from the hand of the avenger of blood, and the congregation shall return him to the city of refuge where he had fled, and he shall remain there until the death of the high priest who was anointed with the holy oil. But if the manslayer at any time goes outside the limits of the city of refuge where he fled, and the avenger of blood finds him outside the limits of his city of refuge, and the avenger of blood kills the manslayer, he shall not be guilty of blood, because 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 may return to the land of his possession.