Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[363] But David, being desirous of ordaining his son king of all the people, called together their rulers to Jerusalem, with the priests and the Levites; and having first numbered the Levites, he found them to be thirty-eight thousand, from thirty years old to fifty; out of which he appointed twenty-three thousand to take care of the building of the temple, and out of the same, six thousand to be judges of the people and scribes, four thousand for porters to the house of God, and as many for singers, to sing to the instruments which David had prepared, as we have said already. He divided them also into courses: and when he had separated the priests from them, he found of these priests twenty-four courses, sixteen of the house of Eleazar, and eight of that of Ithamar; and he ordained that one course should minister to God eight days, from sabbath to sabbath. And thus were the courses distributed by lot, in the presence of David, and Zadok and Abiathar the high priests, and of all the rulers; and that course which came up first was written down as the first, and accordingly the second, and so on to the twenty-fourth; and this partition hath remained to this day. He also made twenty-four parts of the tribe of Levi; and when they cast lots, they came up in the same manner for their courses of eight days. He also honored the posterity of Moses, and made them the keepers of the treasures of God, and of the donations which the kings dedicated. He also ordained that all the tribe of Levi, as well as the priests, should serve God night and day, as Moses had enjoined them.