Abu Zabyan said: I saw Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) at a place called al-Rahbah; he urinated standing until it frothed. Then a vessel of water was brought, and he washed his hands, sniffed water into his nose and rinsed his mouth, then washed his face and forearms and wiped his head. Then he took a palmful of water and placed it on his head, until I saw the water running down onto his beard. Then he wiped over his sandals. Then the iqamah for the prayer was called, so he took off his sandals, stepped forward and led the people in prayer. (b) Al-A'mash said: I told Ibrahim, and he said: When you see Abu Zabyan, tell me. I saw him in al-Kunasah and told him, and he asked him about the hadith. (c) It is reported from Sayyiduna Ali (may Allah be pleased with him) that when he described the ablution of the Messenger of Allah (peace and blessings be upon him), he washed the feet, and he would not go against the way of the Prophet (peace and blessings be upon him). His wiping over the sandals is understood to mean that he had washed his feet with the sandals on. Wiping is a concession for one who has covered his feet with khuffs (leather socks), so it is not extended beyond its place. The basic rule is that washing the feet is obligatory, except in cases covered by an established sunnah or a consensus (ijma'), and wiping over sandals and socks is not permissible.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1366