Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 530
'Urwah al Mughira reported it from his father: He (Mughira) helped the Apostle (may peace be upon him) 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.