(a) Sayyiduna Ali (may Allah be pleased with him) says: Wash the feet up to the ankles, just as you have been commanded.
(b) Sayyiduna Amr bin Abasah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) concerning ablution that he washed his feet up to the ankles just as Allah Almighty commanded concerning it. This is evidence that Allah commanded the washing of the feet.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 338