Numbers 8:10-14

10and thou shalt present the Levites before Jehovah. And the children of Israel shall lay their hands upon the Levites: 11and Aaron shall offer the Levites before Jehovah for a wave-offering, on the behalf of the children of Israel, that it may be theirs to do the service of Jehovah. 12And the Levites shall lay their hands upon the heads of the bullocks: and offer thou the one for a sin-offering, and the other for a burnt-offering, unto Jehovah, to make atonement for the Levites. 13And thou shalt set the Levites before Aaron, and before his sons, and offer them for a wave-offering unto Jehovah.

14Thus shalt thou separate the Levites from among the children of Israel; and the Levites shall be mine.