1 Kings 8:3

The priests took up the ark The ark had been carried by the priests three times before this; when they went over Jordan; when they encompassed the walls of Jericho; and when David sent it back by Zadok and Abiathar, at the time when he fled from Absalom. It was, however, the office of the Levites to carry the ark, which they did, except upon special occasions, of which this was one. The priests were now appointed to carry it for the greater honour of the solemnity; and because the Levites might not enter into the holy place, much less into the holy of holies, where it was to be placed, into which the priests themselves might not have entered, if the high-priest alone could have done this work without them.