It is narrated from Sayyiduna Umar (may Allah be pleased with him) that once the Prophet (peace and blessings be upon him) saw a man who was performing ablution for the Zuhr prayer. While making ablution, he left a spot on the back of his foot the size of a fingernail, meaning he did not wash it or water did not reach there. The Prophet (peace and blessings be upon him) saw him and said, "Go and perform ablution properly." So he went, performed ablution again, and offered the prayer.