Numbers 8:21-26

21 The Levites purified themselves from sin, and they washed their clothes; and Aharon offered them for a wave offering before the LORD; and Aharon made atonement for them to cleanse them.
22 After that, the Levites went in to do their service in the Tent of Meeting before Aharon, and before his sons: as the LORD had commanded Moshe concerning the Levites, so they did to them.
23 The LORD spoke to Moshe, 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 keep the charge, and shall do no service. Thus shall you do to the Levites concerning their duties."
The Hebrew Names Version is in the public domain.