1 Chronicles 23:30

For by the last words of David the sons of Levi were counted, from twenty years old and upward.

For their duty was to wait on the sons of Aaron for the service of the LORD’s house—in the courts, in the rooms, and in the purifying of all holy things, even the work of the service of God’s house;

for the show bread also, and for the fine flour for a meal offering, whether of unleavened wafers, or of that which is baked in the pan, or of that which is soaked, and for all measurements of quantity and size;

and to offer all burnt offerings to the LORD on the Sabbaths, on the new moons, and on the set feasts, in number according to the ordinance concerning them, continually before the LORD;

and that they should keep the duty of the Tent of Meeting, the duty of the holy place, and the duty of the sons of Aaron their brothers for the service of the LORD’s house.