Deuteronomy 19:9

If thou shalt keep all these commandments to do them, which I command thee this day, to love the Lord thy God, and to walk ever in his ways; then shalt thou add three cities more for thee, beside these three:

Cross-reference

Deuteronomy 11:22-25 spells out the same conditional promise: obedience leads to land expansion, which triggers the need for additional refuge cities.

Joshua 20:7 records the actual appointment of the three refuge cities west of the Jordan — the historical fulfillment of the command here.

Joshua 20:8 records the three refuge cities east of the Jordan — the counterpart to the west side, completing the commanded system.

Exodus 21:13 introduces the concept of a place of refuge for accidental killing — the foundational law that the cities of refuge fulfill.