Musnad Ahmad 134
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 saw him and said:
`Go back and do your wudoo’ properly.” So he went back and did wudoo’ then he prayed.
حَدَّثَنَا مُوسَى بْنُ دَاوُدَ، حَدَّثَنَا ابْنُ لَهِيعَةَ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، رَضِيَ اللَّهُ عَنْهُ أَخْبَرَهُ أَنَّهُ، رَأَى رَجُلًا تَوَضَّأَ لِلصَّلَاةِ فَتَرَكَ مَوْضِعَ ظُفُرٍ عَلَى ظَهْرِ قَدَمِهِ فَأَبْصَرَهُ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ ارْجِعْ فَأَحْسِنْ وُضُوءَكَ فَرَجَعَ فَتَوَضَّأَ ثُمَّ صَلَّى‏.‏