عَنْ جَابِرِ بْنِ عَبْدِ اللَّهِ رَضِيَ اللَّهُ عَنْهُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ أَخْبَرَهُ أَنَّهُ رَأَى رَجُلًا تَوَضَّأَ فَتَرَكَ مَوْضِعَ ظُفْرٍ عَلَى ظَهْرِ قَدَمِهِ، فَأَبْصَرَهُ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ فَقَالَ: ((ارْجِعْ فَأَحْسِنْ وَضُوءَكَ)) فَرَجَعَ فَتَوَضَّأَ ثُمَّ صَلَّى
It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) that Sayyiduna Umar bin Khattab (may Allah be pleased with him) saw a man who had performed ablution, but a spot the size of a fingernail on the back of his foot remained dry. The Noble Prophet (peace and blessings be upon him) saw him and said: "Go back and perform ablution properly." So he went back, performed ablution again, and then came and offered prayer.