(5) باب المسح على الخفين
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)].
Reference: Bulugh al-Maram 60
In-book reference: Book 1, Hadith 70
English translation: Book 1, Hadith 65