Mishkat al-Masabih 3 Purification كتاب الطهارة 223

وَعَن عَليّ رَضِي الله عَنهُ قَالَ: لَوْ كَانَ الدِّينُ بِالرَّأْيِ لَكَانَ أَسْفَلُ الْخُفِّ أَوْلَى بِالْمَسْحِ مِنْ أَعْلَاهُ وَقَدْ رَأَيْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَمْسَحُ على ظَاهر خفيه رَوَاهُ أَبُو دَاوُد للدارمي مَعْنَاهُ

‘Ali said, “If the religion were based on opinion, it would be more important to wipe the under part of the shoe than the upper, but I have seen God’s messenger wiping over the upper part of his shoes.” Abu Dawud transmitted it, and Darimi has something similar.