Bulugh al-Maram 60
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)].
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)‏.‏