Numbers 8:15
And after that shall the Levites go in to do the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them for an offering. And after that shall the Levites go in to do the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them for an offering.
Numbers 8:15 belongs to the laws for the purification of the Levites. Their service in the tabernacle of the congregation came after their purification and their presentation as an offering, so they entered that work only after they had been cleansed and offered or set apart for it. That cleansing and setting apart to serve at the tabernacle gave Levi a share of Israel's divided birthright, namely the family priesthood.
The verse is part of the Mosaic instructions governing the Levites' service. Those instructions were followed by the returned Jews when they appointed the Levites to their courses for service in the temple at Jerusalem.