Numbers 8:20-26 - The Levites Are Dedicated To The Lord
20 Moses, 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. 21 The 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. 22 After 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.
23 The LORD spoke to Moses, saying, 24 "This is that which belongs 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; 25 and from the age of fifty years they shall cease waiting on the work, and shall serve no more, 26 but shall minister with their brothers in the Tent of Meeting, to perform the duty, and shall do no service. You shall do thus to the Levites concerning their duties."