Leviticus 16:2-3

16:2 And the LORD said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy [place] within the vail before the mercy seat, which [is] upon the ark; that he die not: for I will appear in the cloud upon the mercy seat. 16:3 Thus shall Aaron come into the holy [place]: with a young bullock for a sin offering, and a ram for a burnt offering.