Sahih Muslim 274g
Sahih Muslim · Book 2: The Book of Purification · Wiping over the khuff (leather socks)
'Urwah al Mughira reported it from his father:
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ ، حَدَّثَنَا إِسْحَاقُ بْنُ مَنْصُورٍ ، حَدَّثَنَا عُمَرُ بْنُ أَبِي زَائِدَةَ ، عَنِ الشَّعْبِيِّ ، عَنْ عُرْوَةَ بْنِ الْمُغِيرَةِ ، عَنْ أَبِيهِ ، أَنَّهُ وَضَّأَ النَّبِيَّ صلى الله عليه وسلم فَتَوَضَّأَ وَمَسَحَ عَلَى خُفَّيْهِ فَقَالَ لَهُ فَقَالَ " إِنِّي أَدْخَلْتُهُمَا طَاهِرَتَيْنِ " .
He (Mughira) helped the Apostle (ﷺ) in performing the ablution, and he performed it and wiped over his shoes. He (Mughira) said to him (about the washing of the feet after putting them off), but he (the Holy Prophet) said: I put them (feet) in when these were clean.