Numbers 8:20-26 - The Levites Are Dedicated To The Lord
20 So Moses and Aaron and the Israelis did this on behalf of the descendants of Levi. The Israelis did everything that the LORD commanded concerning the descendants of Levi. 21 The descendants of Levi therefore purified themselves, washed their clothes, and then Aaron presented them as wave offerings to the LORD. Aaron provided atonement for them to purify them. 22 After this, the descendants of Levi entered into their work of service at the appointed place, in the presence of Aaron and his sons. They did everything that the LORD commanded Moses concerning the descendants of Levi.
23 Later, the LORD told Moses, 24 "Now regarding a descendant of Levi who is 25 years and above, he is to enter work in the service at the appointed place of meeting, 25 but starting at 50 years of age, he is to retire from service and is no longer to work. 26 He may minister to his brothers at the Tent of Meeting by keeping watch, but he is not to engage in service. This is how you are to act with respect to the obligations of the descendants of Levi."