Deuteronomy 19:8-9 -

In case their land should be extended, in ease they should come to possess the whole territory promised by God to the patriarchs, so that their domain should reach from the Nile to the Euphrates ( Genesis 15:18 )—an event which should be realized only if they should continue steadfast in their obedience to all that God had enjoined upon them, and an event which in point of fact never was realized, for even under David and Solomon there were extensive territories within these limits which were not incorporated with the kingdom of Israel—in that case they were to add other three cities of refuge to those already appointed.