Leviticus 16:1-3 ESV
The LORD spoke to Moses after the death of the two sons of Aaron, when they drew near before the LORD and died, and the LORD said to Moses, “Tell Aaron your brother not to come at any time into the Holy Place inside the veil, before the mercy seat that is on the ark, so that he may not die. For I will appear in the cloud over the mercy seat. But in this way Aaron shall come into the Holy Place: with a bull from the herd for a sin offering and a ram for a burnt offering.
ESV: English Standard Version 2016