Numbers 20:23

על גבול ארץ אדום BY THE BOUNDARY OF THE LAND OF EDOM — This (the statement that Aaron died here) tells us that because they here wished to join themselves in close friendship with the wicked, a breach was made in their works and they had to lose this righteous man, Aaron. Similarly does the prophet say, (II Chronicles 20:37) “Because thou hast joined thyself with Ahaziah, the Lord hath made a breach in thy works” (Midrash Tanchuma, Chukat 14).

וַיֹּ֧אמֶר יְהֹוָ֛ה אֶל־מֹשֶׁ֥ה וְאֶֽל־אַהֲרֹ֖ן בְּהֹ֣ר הָהָ֑ר עַל־גְּב֥וּל אֶֽרֶץ־אֱד֖וֹם לֵאמֹֽר׃

And the Lord spoke to Moshe and Aharon in mount Hor, by the border of the land of Edom, saying,