It is narrated from Sayyiduna Umar (may Allah be pleased with him) that once he saw a person who was performing ablution for prayer, and while doing so, he left an area 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) also saw him and said, "Go and perform ablution properly." So he went, performed ablution again, and offered the prayer.