Numbers 4:21-23

Young's Literal Translation

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.