Leviticus 16:1-3

Now the LORD spoke to Moses after the death of the two sons of Aaron, when they had approached the presence of the LORD and died. The LORD said to Moses: “ T ell your brother Aaron that he shall not enter at any time into the holy place inside the veil, before the mercy seat which is on the ark, or he will die; for I will appear in the cloud over the mercy seat. Aaron shall enter the holy place with this: with a bull for a sin offering and a ram for a burnt offering.