Numbers 8:15-8:26

Thereafter the Levites shall be qualified for the service of the Tent of Meeting, once you have cleansed them and designated them as an elevation offering. For they are formally assigned to Me from among the Israelites: I have taken them for Myself in place of all the first issue of the womb, of all the first-born of the Israelites. For every first-born among the Israelites, man as well as beast, is Mine; I consecrated them to Myself at the time that I smote every first-born in the land of Egypt. Now I take the Levites instead of every first-born of the Israelites; and from among the Israelites I formally assign the Levites to Aaron and his sons, to perform the service for the Israelites in the Tent of Meeting and to make expiation for the Israelites, so that no plague may afflict the Israelites for coming too near the sanctuary. Moses, Aaron, and the whole Israelite community did with the Levites accordingly; just as the Lord had commanded Moses in regard to the Levites, so the Israelites did with them. The Levites purified themselves and washed their clothes; and Aaron designated them as an elevation offering before the Lord, and Aaron made expiation for them to cleanse them. Thereafter the Levites were qualified to perform their service in the Tent of Meeting, under Aaron and his sons. As the Lord had commanded Moses in regard to the Levites, so they did to them. The Lord spoke to Moses, saying: This is the rule for the Levites. From twenty-five years of age up they shall participate in the work force in the service of the Tent of Meeting; but at the age of fifty they shall retire from the work force and shall serve no more. They may assist their brother Levites at the Tent of Meeting by standing guard, but they shall perform no labor. Thus you shall deal with the Levites in regard to their duties.