Numbers 8:8

8:8 Then they are to take a young bull with its grain offering of fine flour mixed with olive oil; and you are to take a second young bull for a purification offering.

Numbers 8:12

8:12 When the Levites lay their hands on the heads of the bulls, offer the one for a purification offering and the other for a whole burnt offering to the Lord, to make atonement for the Levites.