Sunan at-Tirmithiy كتاب الطهارة باب المسح على الخفين للمسافر والمقيم
The Prophet, peace and blessings of God be upon him, was asked about wiping over (1) one's footwear (2) [in ablution, instead of washing one's feet], so he said, "The traveler [is allowed to do so for] three days, the resident one day." Footnotes: (1) The period of time during which one may do this. (2) Khuff:: Footwear that is made of leather etc. It covers the ankles and a portion above it. It cannot be transparent, and it remains attached to the foot preventing water from going through. Click on ḥadīth to read more