Numbers 8:7-11

This is what you shall do to them, for their cleansing: sprinkle purifying water on them, and have them use a razor over their whole body, and they shall wash their clothes and cleanse themselves.

Then have them take a bull with its grain offering, fine flour mixed with oil; and you shall take a second bull as a sin offering.

So you shall present the Levites in front of the tent of meeting. You shall also assemble the whole congregation of the sons of Israel,

and present the Levites before the Lord; and the sons of Israel shall lay their hands on the Levites.

Aaron then shall present the Levites before the Lord as a wave offering from the sons of Israel, so that they may qualify to perform the service of the Lord.