1 Chronicles 23:3-6

3And the Levites were numbered from thirty years old and upward: and their number by their polls, man by man, was thirty and eight thousand. 4Of these, twenty and four thousand were to oversee the work of the house of Jehovah; and six thousand were officers and judges; 5and four thousand were doorkeepers; and four thousand praised Jehovah with the instruments which I made, said David, to praise therewith. 6And David divided them into courses according to the sons of Levi: Gershon, Kohath, and Merari.