John Gill Commentary Joshua 20:2

"Speak to the children of Israel, saying, Assign you the cities of refuge, whereof I spake unto you by Moses," — Joshua 20:2 (ASV)

Speak to the children of Israel, saying
Whom the affair concerned the Lord spoke to Joshua about, they having now the whole land divided among them:

appoint out for you cities of refuge ;
of the name, nature, use, and number of these cities, (See Gill on Numbers 35:6);

whereof I spoke to you by the hand of Moses :
(Numbers 35:1–34) (Deuteronomy 19:1–21) .