Numbers 8:22
And after that went the Levites in to do their service in the tabernacle of the congregation before Aaron, and before his sons: as the LORD had commanded Moses concerning the Levites, so did they unto them. And after that went the Levites in to do their service in the tabernacle of the congregation before Aaron, and before his sons: as the LORD had commanded Moses concerning the Levites, so did they unto them.
After their consecration the Levites went in to perform their appointed service in the tabernacle of the congregation under the direction of Aaron and his sons. What was done to them and by them was done exactly as the Lord had commanded Moses concerning the Levites. That commanded entrance into service stands as the divinely given pattern for appointing the Levites to their courses for God's service at Jerusalem.