‏ Numbers 8:11-12

11and Aaron shall offer the Levites before the Lord as a wave offering from the people of Israel, that they may do the service of the Lord. 12Then the Levites ashall lay their hands on the heads of the bulls, and you shall offer bthe one for a sin offering and the other for a burnt offering to the Lord to make atonement for the Levites.
Copyright information for ESV