Abdu Khair (may Allah have mercy on him) says that I once saw Sayyiduna Ali (may Allah be pleased with him) performing ablution; he wiped over his sandals and said: If I had not seen the Prophet (peace and blessings be upon him) washing the upper part of the foot, I would have thought that the lower part of the foot was more deserving of being washed (because it touches the ground more).