1 Chronicles 23:2-6

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