Midrash su I Cronache 15:11

(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."

(Bamidbar 7:9) "And to the sons of Kehath he gave no (wagons)": R. Nathan says: This escaped David, that the Levites bore the ark in a wagon, as it is written (I Samuel 6:3) "And they placed the ark of G-d on a new wagon … (7) And the L-rd was wroth with Uzzah, and He smote him there for erring … (8) And David grieved over the L-rd's having made a breach in Uzzah." Achitofel said to David: Should you not have learned from Moses your master that the Levites bore the ark only on their shoulder, as it is written "And to the sons of Kehath he gave none for the burden of the holy things was theirs, (wherefore) they were to be borne upon the shoulder"? Therefore, David afterwards sent and had it borne on the shoulder, as it is written (I Chronicles 15:11-15) "And David called to Tzaddok and to Evyathar the Cohanim, and to the Levites: to Uriel, Assayah, and Yoel, Shemayah, Eliaz, and Aminadav. And he said to them: You are the heads of the fathers' (houses) of the Levites. Ready yourselves and your brothers, and you shall bring up the ark of the L-rd, the G-d of Israel. For in the beginning (when the ark was brought up from Kiryat Yearim), it was not you (who were the bearers, wherefore) the L-rd our G-d made a breach in us … And the Cohanim and the Levites readied themselves … And the sons of the Levites bore the ark of G-d as Moses had commanded by word of the L-rd, on their shoulders, with staves upon them." And where did he so command? "And to the sons of Kehath he gave none, etc." (Ibid. 24:19 ["These are their numbers (i.e., the numbers of their watches) for their service, to come to the house of the L-rd as ordained, by the hand of Aaron, their father, as the L-rd, the G-d of Israel, commanded him."]) They originated nothing, but (did) all from the mouth of Moses, and Moses from the mouth of the Omnipotent.