John Gill Commentary Numbers 8:14

"Thus shalt thou separate the Levites from among the children of Israel; and the Levites shall be mine." — Numbers 8:14 (ASV)

Thus shall you separate the Levites from among the children
of Israel
By sprinkling the water of purification upon them; by offering sacrifices for them; by presenting them before the Lord, with the hands of the people of Israel put upon them; by offering and waving them before the Lord, and by setting them before Aaron and his sons, as their ministers:

and the Levites shall be mine :
in a special and peculiar manner devoted to his service, and by him given to his priests for that purpose; so the Targums of Onkelos and Jonathan, ``and the Levites shall minister before me.''