Bulugh al-Maram 1 The Book of Purification كتاب الطهارة 70

وَعَنْ عَلِيٍّ – رضى الله عنه – قَالَ: { لَوْ كَانَ اَلدِّينُ بِالرَّأْيِ لَكَانَ أَسْفَلُ اَلْخُفِّ أَوْلَى بِالْمَسْحِ مِنْ أَعْلَاهُ, وَقَدْ رَأَيْتُ رَسُولَ اَللَّهِ – صلى الله عليه وسلم -يَمْسَحُ عَلَى ظَاهِرِ خُفَّيْهِ } أَخْرَجَهُ أَبُو دَاوُدَ بِإِسْنَادٍ حَسَن ٍ 1 . 1 – صحيح. رواه أبو داود (162).

Narrated ‘Ali (rad): 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 Allah’s Messenger (rad) wiping over the upper parts of his leather socks. [Reported by Abu Da’ud with good Isnad (chain of narrators)].