Sayyiduna Jabir (may Allah be pleased with him) says that Umar bin al-Khattab (may Allah be pleased with him) saw a man performing ablution, and a spot on his foot remained dry. Sayyiduna Umar (may Allah be pleased with him) said: Repeat the ablution.
(b) It is narrated from Hadrat Umar (may Allah be pleased with him) that the order he gave to repeat the ablution was recommended, and only washing the foot was obligatory.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 395