1 Chronicles 23:6, 25-31

And David divided them into courses among the sons of Levi, namely, Gershon, Kohath, and Merari.

For David said, The LORD God of Israel hath given rest unto his people, that they may dwell in Jerusalem for ever: and also unto the Levites; they shall no more carry the tabernacle, nor any vessels of it for the service thereof. For by the last words of David the Levites were numbered from twenty years old and above: because their office was to wait on the sons of Aaron for the service of the house of the LORD, in the courts, and in the chambers, and in the purifying of all holy things, and the work of the service of the house of God; both for the shewbread, and for the fine flour for meat offering, and for the unleavened cakes, and for that which is baked in the pan, and for that which is fried, and for all manner of measure and size; and to stand every morning to thank and praise the LORD, and likewise at even; and to offer all burnt sacrifices unto the LORD in the sabbaths, in the new moons, and on the set feasts, by number, according to the order commanded unto them, continually before the LORD