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. (Using translation from Abū Dāʾūd 175)
عَنْ بَعْضِ أَصْحَابِ النَّبِيِّ ﷺ أَنَّ رَسُولَ اللهِ ﷺ رَأَى رَجُلًايُصَلِّي وَفِي ظَهْرِ قَدَمِهِ لُمْعَةٌ قَدْرُ الدِّرْهَمِ لَمْ يُصِبْهَا الْمَاءُ فَأَمَرَهُ رَسُولُ اللهِ ﷺ أَنْ يُعِيدَ الْوُضُوءَ