Muwatta Malik 75

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، أَنَّهُ رَأَى أَبَاهُ يَمْسَحُ عَلَى الْخُفَّيْنِ قَالَ وَكَانَ لاَ يَزِيدُ إِذَا مَسَحَ عَلَى الْخُفَّيْنِ عَلَى أَنْ يَمْسَحَ ظُهُورَهُمَا وَلاَ يَمْسَحُ بُطُونَهُمَا ‏.‏

Yahya related to me from Malik from Hisham ibn Urwa that he saw his father wiping over his leather socks. He said, "When he wiped over his socks he would never do more than wipe the tops and he would not wipe the bottoms

Reference

Muwatta Malik 75

In-book

Book 2, Hadith 44