‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) saw a man who had just finished performing ablution leaving a spot on his foot, the size of a fingernail, unwashed. He said to him while pointing to the place that was not properly washed: Go back and perform your ablution properly and perfectly and wash each organ properly with water. So, the man went back, performed ablution perfectly, and then prayed.