Numbers 35:26-28

26But if the slayer shall at any time come outside the border of the city of his refuge, where he had fled; 27And the avenger of blood find him outside the borders of the city of his refuge, and the avenger of blood shall kill the slayer; he shall not be guilty of blood:1 28Because he should have remained in the city of his refuge until the death of the high priest: but after the death of the high priest the slayer shall return into the land of his possession.