Among the obligatory acts of ablution is washing the feet up to the ankles once: as His Exalted says: (AND YOUR FEET TO THE ANKLES) with the "lam" in "الأرجل" (the feet) being in the accusative case, in conjunction with His Exalted saying: (THEN WASH YOUR FACES AND YOUR HANDS TO THE ELBOWS) Al-Ma'idah: 6, as if He said: wash your faces, and your hands to the elbows, and your feet to the ankles, and wipe over your heads, and what is conjoined with the washed must be washed in accordance with the requirement of conjunction. And from Abdullah ibn Zaid, may Allah be pleased with him, he said: ((The Messenger of Allah, peace be upon him... performed ablution, washing his face three times, his hands twice, wiping over his head, and washing his feet)) in Sahih Muslim 1: 211, and Sahih Bukhari 1: 83. And from Ali, may Allah be pleased with him, he said: ((He performed ablution, rinsing his mouth three times, inhaling water three times from one hand, washing his face three times, then he put his hand in the basin and wiped his head and washed his feet, then he said this is the ablution of your Prophet, peace be upon him)) in Al-Athar 1: 2, and Musnad Abu Hanifa p. 99, and Musannaf Ibn Abi Shaybah 1: 16. And from Ata, that Uthman, may Allah be pleased with him: ((performed ablution three times, and wiped over his head once, and washed his feet, then he said: this is how I saw the Messenger of Allah, peace be upon him, perform ablution)) in Musannaf Ibn Abi Shaybah 1: 17. See: Bada'i' al-Sana'i 1: 5. And Allah knows best.