Numbers 8:15-26

Then after that the Levites may go in to serve the Tent of Meeting (tabernacle), but you shall cleanse them [first] and present them as a wave offering; for they are wholly given to Me from among the Israelites. I have taken them for Myself instead of all who are born first, the firstborn of all the Israelites. For all the firstborn among the Israelites are Mine, among the men and among the animals; on the day that I struck down every firstborn in the land of Egypt, I sanctified and set the Israelites apart for Myself. And I have taken the Levites instead of every firstborn among the Israelites. I have given the Levites as a gift to Aaron and to his sons from among the Israelites, to perform the service of the Israelites at the Tent of Meeting (tabernacle) and to make atonement for them, so that there will be no plague among the Israelites if they should approach the sanctuary.” Thus Moses and Aaron and all the congregation of the children of Israel did [these things] to the Levites; in accordance with all that the LORD had commanded Moses concerning the Levites, so the Israelites did to them. The Levites, too, purified themselves from sin and they washed their clothes; and Aaron presented them as a wave offering before the LORD, and Aaron made atonement for them to cleanse them. Then after that the Levites went in to perform their service in the Tent of Meeting before Aaron and his sons; just as the LORD had commanded Moses concerning the Levites, so they did to them. Now the LORD spoke to Moses, saying, “This is what applies to the Levites: from twenty-five years old and upward they shall enter to perform service in the work of the Tent of Meeting, but at the age of fifty years, they shall retire from the service of the [tabernacle] work and serve no longer. They may assist their brothers in the Tent of Meeting to keep an obligation, but they shall do no [heavy or difficult] work. Thus you shall deal with the Levites concerning their obligations.”