Umar bin Khattab informed me that a man performed ablution but left a spot the size of a fingernail on his foot untouched. The Prophet ﷺ noticed it and said, "Go back and perform your ablution properly." So the man went back, then prayed.
أخبرني عمر بن الخطاب أن رجلا توضأ فترك موضع ظفر على قدمه، فأبصره النبي ﷺ فقال: ارجع! فأحسن وضوءك. فرجع ثم صلى.