Narrated Some Companions of the Prophet:

The Prophet ﷺ saw a person offering prayer, and on the back of his foot a small part equal to the space of a dirham remained unwashed; the water did not reach it. The Prophet ﷺ commanded him to repeat the ablution and prayer.

حَدَّثَنَا حَيْوَةُ بْنُ شُرَيْحٍ، حَدَّثَنَا بَقِيَّةُ، عَنْ بَحِيرٍ، - هُوَ ابْنُ سَعْدٍ - عَنْ خَالِدٍ، عَنْ بَعْضِ، أَصْحَابِ النَّبِيِّ أَنَّ النَّبِيَّ ﷺ رَأَى رَجُلاً يُصَلِّي وَفِي ظَهْرِ قَدَمِهِ لُمْعَةٌ قَدْرُ الدِّرْهَمِ لَمْ يُصِبْهَا الْمَاءُ فَأَمَرَهُ النَّبِيُّ ﷺ أَنْ يُعِيدَ الْوُضُوءَ وَالصَّلاَةَ .