Numbers 35:6-8

6“Six of the cities you give the Levites will be cities of refuge. You must allow murderers to escape to these cities. In addition, you must also give the Levites 42 other cities.

7So you will give a total of 48 cities with pastureland to the Levites.

8The cities you give the Levites from the property of the other Israelites must be given based on the amount of land each tribe owns. Take more cities from larger tribes and fewer from smaller tribes.”