Leviticus 16:1-3

Then the LORD spoke to Moses after the death of the two sons of Aaron who had died when they [irreverently] approached the presence of the LORD. [Lev 10:1, 2] The LORD said to Moses, “ T ell Aaron your brother that he must not enter at any time into the Holy Place inside the veil (the Holy of Holies), before the mercy seat which is on the ark, or he will die, for I will appear in the cloud over the mercy seat. [Heb 9:7-15, 25-28] Aaron [as high priest] shall enter the Holy Place in this way: with [the blood of] a young bull as a sin offering and [the blood of] a ram as a burnt offering.