وَعَنْ عَلِيٍّ رضى الله عنه
قَالَ: «لَوْ كَانَ اَلدِّينُ بِالرَّأْيِ لَكَانَ أَسْفَلُ اَلْخُفِّ أَوْلَى بِالْمَسْحِ مِنْ أَعْلَاهُ، وَقَدْ رَأَيْتُ رَسُولَ اَللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَمْسَحُ عَلَى ظَاهِرِ خُفَّيْهِ»
If the religion were based on opinion, it would be more important to wipe over the under parts of the leather socks than the upper, but I have seen the Messenger of Allah (sallallahu alayhi wa sallam) (rad) wiping over the upper parts of his leather socks.
Bulugh al-Maram 70, Book 1, The Book of Purification, Hadith 70. 1 min read