Numbers 4:19-23

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;

From thirty years old and upward until fifty years old shalt thou number them; all that enter in to perform the service, to do the work in the tabernacle of the congregation.