Hadith 1:175
Sahih (Al-Albani)
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 صلى الله عليه وسلم رَأَى رَجُلاً يُصَلِّي وَفِي ظَهْرِ قَدَمِهِ لُمْعَةٌ قَدْرُ الدِّرْهَمِ لَمْ يُصِبْهَا الْمَاءُ فَأَمَرَهُ النَّبِيُّ صلى الله عليه وسلم أَنْ يُعِيدَ الْوُضُوءَ وَالصَّلاَةَ
Original Reference: Sunan Abi Dawud 175