Numbers 4:17-20

And the Lord said to Moses and Aaron, Do not let the family of the Kohathites be cut off from among the Levites; But do this to them, so that life and not death may be theirs when they come near the most holy things; let Aaron and his sons go in and give to every one his work and that which he is to take up; But they themselves are not to go in to see the holy place, even for a minute, for fear of death.