مسند أحمد

It was narrated from Jabir that 'Umar bin al-Khattab رضي الله عنه told him that he saw a man doing wudoo’ for prayer, and he missed an area the size of a fingernail on the top of his foot. The Prophet (ﷺ) ﷺ saw him and said: `Go back and do your wudoo’ properly.” So he went back and did wudoo’ then he prayed.