Numbers 8:20

16For they are wholly given to me from among the children of Israel; instead of all who open the womb, even the firstborn of all the children of Israel, I have taken them to me.

17For all the firstborn among the children of Israel are mine, both man and animal. On the day that I struck all the firstborn in the land of Egypt, I sanctified them for myself.

18I have taken the Levites instead of all the firstborn among the children of Israel.

19I have given the Levites as a gift to Aaron and to his sons from among the children of Israel, to do the service of the children of Israel in the Tent of Meeting, and to make atonement for the children of Israel, so that there will be no plague among the children of Israel when the children of Israel come near to the sanctuary.”

20Moses, and Aaron, and all the congregation of the children of Israel did so to the Levites. According to all that the LORD commanded Moses concerning the Levites, so the children of Israel did to them.

21The Levites purified themselves from sin, and they washed their clothes; and Aaron offered them for a wave offering before the LORD and Aaron made atonement for them to cleanse them.

22After that, the Levites went in to do their service in the Tent of Meeting before Aaron and before his sons: as the LORD had commanded Moses concerning the Levites, so they did to them.

23The LORD spoke to Moses, saying,

24“This is what is assigned to the Levites: from twenty-five years old and upward they shall go in to wait on the service in the work of the Tent of Meeting;