Leviticus 16:2-3

2and the Lord said to Moses: “Tell Aaron your brother that he must not enter at any time into the holy place inside the veil-canopy in front of the atonement plate that is on the ark so that he may not die, for I will appear in the cloud over the atonement plate.

3“In this way Aaron is to enter into the sanctuary – with a young bull for a sin offering and a ram for a burnt offering.