Numbers 4:18 (KJV)

And to the office of Eleazar the son of Aaron the priest pertaineth the oil for the light, and the sweet incense, and the daily meat offering, and the anointing oil, and the oversight of all the tabernacle, and of all that therein is, in the sanctuary, and in the vessels thereof.

And the Lord spake unto Moses and unto Aaron, saying,

Cut ye not off the tribe of the families of the Kohathites from among the Levites:

But thus do unto them, that they may live, and not die, when they approach unto the most holy things: Aaron and his sons shall go in, and appoint them every one to his service and to his burden:

But they shall not go in to see when the holy things are covered, lest they die.