Midrasch zu Schemot 26:32

Observe that many miracles were performed by means of the ark. Since that is so, how were they able to carry it? Were they not afraid? That is why the sons of Kohath bore it. Two important priests bore the ark cover on a pole.¹³Numbers Rabbah 4:13. See Exod. 26:32. It was a huge curtain the thickness of a handbreadth. The curtain was woven out of seventy-two strands, and each strand was composed of twenty-four threads. Three hundred priests would wash it, and two of the highest priests would bear it on a pole before the ark. Then they placed a sealskin covering over the ark so that they would not be able to see into it, as it is said: But they shall not go in to see the holy things as they are being covered, lest they die (Num. 4:20).