Midrasch zu Divrej Hajamim II 20:35

16 (Numb. 20:22) “Then setting out from Kadesh, the whole congregation [of the Children of Israel came to Mount Hor]”: This text is related (to II Chron. 20:37), “Because you have joined with Ahaziah,⁶⁵Son of Ahab and a wicked king of Israel (I Kings 22:51-52), with whom Jehoshaphat had allied himself (I Kings 22:44; II Chron. 20:35.) the Lord will destroy your work.” [Similarly,] because they made an alliance with this wicked king to pass through his land, they lost this righteous man (i.e., Aaron). For that reason the death of Aaron⁶⁶See Numb. 20:28: … AND AARON DIED THERE ON THE TOP OF THE MOUNTAIN. is made [immediately] adjacent after the parashah about the king of Edom. (Numb. 20:21-22), “So Israel turned away from them. Then setting out from Kadesh, [the whole congregation of the Children of Israel came to Mount Hor]”:⁶⁷Cf. yYoma 1:1 (38ab). What is the meaning of “the whole congregation?” A complete congregation, a congregation which would be entering the land, since those who had come out from Egypt had died. So these were the ones of whom it is written (in Deut. 4:4), “But you who clung to the Lord your God are all alive today.” (Numb. 20:22) “Mount (hr) Hor (hr)”: What is [its] meaning? A mountain (hr) on top of a mountain (hr), like a small apple on a large apple. Even though a cloud proceeded before them which lowered the high [places] and raised up the low, the Holy One, blessed be He, left this mountain as a sample,⁶⁸Gk.: deigma. so that they would know what miracles the Holy One, blessed be He, had done for them; as He had not left a mountain in the desert, lest they become weary climbing and descending. Moreover, although the cloud had made all the desert a plain, He left an elevated spot where the tabernacle would have its resting place. He left three mountains: Mount Sinai for the Divine Presence, Mount Nebo for the burial of Moses, and Mount Hor for the burial of Aaron.