Numbers 8:9-12

And thou shalt bring the Levites before the tabernacle of the congregation: and thou shalt gather the whole assembly of the children of Israel together: And thou shalt bring the Levites before the LORD: and the children of Israel shall put their hands upon the Levites: And Aaron shall offer the Levites before the LORD {for} an offering of the children of Israel, that they may execute the service of the LORD. {offer: Heb. wave} {offering: Heb. wave offering} {they...: Heb. they may be to execute, etc} And the Levites shall lay their hands upon the heads of the bullocks: and thou shalt offer the one {for} a sin offering, and the other {for} a burnt offering, unto the LORD, to make an atonement for the Levites.