It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) that Sayyiduna Umar bin Khattab (may Allah be pleased with him) saw a man who had performed ablution, but a spot the size of a fingernail on the back of his foot remained dry. The Noble Prophet (peace and blessings be upon him) saw him and said: "Go back and perform ablution properly." So he went back, performed ablution again, and then came and offered prayer.