Numbers 4:20 (KJV)

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.

And the Lord spake unto Moses, saying,

Take also the sum of the sons of Gershon, throughout the houses of their fathers, by their families;