Мидраш к Бамидбар 7:6

(Bamidbar 7:6) "And Moses took the wagons and the oxen and he gave them to the Levites": Moses took them and distributed them as he saw fit: Two wagons and four oxen he gave to the sons of Gershon, and four wagons and eight oxen he gave to the sons of Merari. And thus do we find with David, that he distributed the heads of the watches among the Cohanim, viz. (I Chronicles 24:4-6) "And there were more chiefs found of the sons of Elazar than of the sons of Ithamar, and thus were they divided. Among the sons of Elazar there were sixteen chiefs of the house of their fathers, and eight among the sons of Ithamar according to the house of their fathers. And they were divided by lot, these with the others … and Shemayah the son of Nathanel, of the Levites, recorded them before the king and the princes, and Tzaddok the Cohein and Achimelech the son of Avyathar, etc."