Maliks Muwatta Book 2, Hadith Number 43.
Section : Wiping over Leather Socks.
Yahya related to me from Malik that Nafi and Abdullah ibn Dinar told him that Abdullah ibn Umar arrived at Kufa and went to Sad ibn Abi Waqqas, who was the Amir of Kufa at that time. Abdullah ibn Umar saw him wiping over his leather socks and disapproved of it. So Sad said to him, “Ask your father when you get back.” Abdullah returned but forgot to ask Umar about the matter until Sad arrived and said, “Have you asked your father?” and he said, “No.”
Abdullah then asked Umar and Umar replied, “If your feet are ritually pure when you put them in the leather socks then you can wipe over the socks.” Abdullah said,”What about if we have just come from relieving ourselves?” Umar said, “Yes, even if you have just come from relieving yourself.”