Numbers 4:21 YLT

¹⁹  but this do to them, and they have lived, and do not die in their drawing nigh the holy of holies: -- Aaron and his sons go in, and have set them, each man to his service, and unto his burden,

²⁰  and they go not in to see when the holy thing is swallowed, that they have died.'

²¹  And Jehovah speaketh unto Moses, saying,

²²  'Take up the sum of the sons of Gershon also by the house of their fathers, by their families;

²³  from a son of thirty years and upward, till a son of fifty years thou dost number them, every one who is going in to serve the host, to do the service in the tent of meeting.