1 Chronicles 23:6, 25-31

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

For David said, “The LORD God of Israel has given rest to His people, and He dwells in Jerusalem forever. Also, the Levites will no longer need to carry the tabernacle and all its utensils for its service.” For by the last words of David the sons of Levi were numbered from twenty years old and upward. For their office is to assist the sons of Aaron with the service of the house of the LORD, 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, and with the showbread, and the fine flour for a grain offering, and unleavened wafers, or what is baked in the pan or what is well-mixed, and all measures of volume and size. They are to stand every morning to thank and to praise the LORD, and likewise at evening, and to offer all burnt offerings to the LORD, on the sabbaths, the new moons and the fixed festivals in the number set by the ordinance concerning them, continually before the LORD.