Musnad Abu Ya'la al-Mawsili 2:32

Abu Kurayb narrated to us, Zayd narrated to us, from Khalid ibn Abi Bakr, Salim narrated to us, from Ibn Umar, from Umar, who said: I heard the Prophet, peace and blessings of Allah be upon him, commanding us to wipe over the leather socks (khuffayn). For the traveler, it is three days and their nights, and for the resident, it is a day and a night.