Numbers 35:26-28

26But if at any time the one guilty of manslaughter comes outside the border of his city of refuge to which he fled,

27and the blood avenger finds him outside the border of his city of refuge and kills the offender, the blood avenger will not be guilty of murder,

28because the offender should have remained in his city of refuge until the death of the high priest. But after the high priest’s death the offender shall return to the land of his possession.