Leviticus 25:32-33

³²And the cities of the Levites, the houses of the cities in their possession, shall be always redeemable to the Levites.

³³And if any one shall redeem a house of the Levites, then shall their sale of the houses of their possession go out in the release; because the houses of the cities of the Levites are their possession in the midst of the children of Israel.