Numbers 4:36

This is the service of the families of the sons of Merari, according to all their service in the Tent of Meeting, under the hand of Ithamar the son of Aaron the priest.”

Moses and Aaron and the princes of the congregation counted the sons of the Kohathites by their families, and by their fathers’ houses,

from thirty years old and upward even to fifty years old, everyone who entered into the service for work in the Tent of Meeting.

These are those who were counted of the families of the Kohathites, all who served in the Tent of Meeting, whom Moses and Aaron counted according to the commandment of the LORD by Moses.

Those who were counted of the sons of Gershon, by their families, and by their fathers’ houses,

from thirty years old and upward even to fifty years old—everyone who entered into the service for work in the Tent of Meeting,