Numbers 8:18-26

18 and I have taken the Levites instead of all the first-born among the people of Israel.
19 And I have given the Levites as a gift to Aaron and his sons from among the people of Israel, to do the service for the people of Israel at the tent of meeting, and to make atonement for the people of Israel, that there may be no plague among the people of Israel in case the people of Israel should come near the sanctuary."
20 Thus did Moses and Aaron and all the congregation of the people of Israel to the Levites; according to all that the LORD commanded Moses concerning the Levites, the people of Israel did to them.
21 And the Levites purified themselves from sin, and washed their clothes; and Aaron offered them as a wave offering before the LORD, and Aaron made atonement for them to cleanse them.
22 And after that the Levites went in to do their service in the tent of meeting in attendance upon Aaron and his sons; as the LORD had commanded Moses concerning the Levites, so they did to them.
23 And the LORD said to Moses,
24 "This is what pertains to the Levites: from twenty-five years old and upward they shall go in to perform the work in the service of the tent of meeting;
25 and from the age of fifty years they shall withdraw from the work of the service and serve no more,
26 but minister to their brethren in the tent of meeting, to keep the charge, and they shall do no service. Thus shall you do to the Levites in assigning their duties."
Revised Standard Version of the Bible, copyright 1952 [2nd edition, 1971] by the Division of Christian Education of the National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved.