1 Chronicles 23:6, 25-31

And David divided them into courses according to the sons of Levi: Gershon, Kohath, and Merari.

For David said, Jehovah, the God of Israel, hath given rest unto his people; and he dwelleth in Jerusalem for ever: and also the Levites shall no more have need to carry the tabernacle and all the vessels of it for the service thereof. For by the last words of David the sons of Levi were numbered, from twenty years old and upward. For their office was to wait on the sons of Aaron for the service of the house of Jehovah, in the courts, and in the chambers, and in the purifying of all holy things, even the work of the service of the house of God; for the showbread 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 manner of measure and size; and to stand every morning to thank and praise Jehovah, and likewise at even; and to offer all burnt-offerings unto Jehovah, on the sabbaths, on the new moons, and on the set feasts, in number according to the ordinance concerning them, continually before Jehovah