Midrasch zu Schemuel I 6:3

(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.