Numbers 35:13

שש ערי מקלט [AND AS TO THE CITIES WHICH YOU SHALL GIVE] SIX CITIES OF REFUGE (THERE SHALL BE FOR YOU] — This tells us that although Moses during his life-time set aside three cities on that side of the Jordan (Deuteronomy 4:41), they did not provide refuge until the three had been chosen which Joshua assigned in the land of Canaan on the west side of the Jordan (Sifrei Bamidbar 160:2; Makkot 9b).

וְהֶעָרִ֖ים אֲשֶׁ֣ר תִּתֵּ֑נוּ שֵׁשׁ־עָרֵ֥י מִקְלָ֖ט תִּהְיֶ֥ינָה לָכֶֽם׃

And the cities which you shall give shall be six cities for refuge.