Musnad Ahmad 5:710

Yazid narrated to us, Hajjaj ibn Artah informed us, from al-Hakam, from al-Qasim ibn Mukhaymirah, from Shurayh ibn Hani, who said: I asked Aisha about wiping over the leather socks (khuffayn). She said: "Ask Ali, for he is more knowledgeable about this than I am; he used to travel with the Messenger of Allah, peace and blessings of Allah be upon him." So I asked Ali, and he said: "The Messenger of Allah, peace and blessings of Allah be upon him, said: 'For the resident, a day and a night, and for the traveler, three days and their nights.'"