Leviticus 16:2 in the King James Version
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.
Context
1 The LORD spoke to Moses after the death of the two sons of Aaron, when they came near before the LORD, and died;
2 and the LORD said to Moses, “Tell Aaron your brother not to come at just any time into the Most Holy Place within the veil, before the mercy seat which is on the ark; lest he die; for I will appear in the cloud on the mercy seat.
3 “Aaron shall come into the sanctuary with a young bull for a sin offering, and a ram for a burnt offering.