A man performed ablution and left a small part on his foot equal to the size of a nail unwashed. The Prophet (may Allah's peace and blessings be upon him) saw that and said: "Go back and perform your ablution properly." He performed it again and prayed

‘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.