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 النَّبِيَّ ﷺ رَأَى رَجُلًا يُصَلِّي وَفِي ظَهْرِ قَدَمِهِ لُمْعَةٌ قَدْرُ الدِّرْهَمِ لَمْ يُصِبْهَا الْمَاءُ فَأَمَرَهُ النَّبِيُّ ﷺ أَنْ يُعِيدَ الْوُضُوءَ وَالصَّلَاةَ كَذَا فِي هَذَا الْحَدِيثِ وَهُوَ مُرْسَلٌ وَرُوِيَ فِي حَدِيثٍ مَوْصُولٍ  

[Machine] A man came to the Prophet ﷺ having performed ablution, but he left a small area on his foot unwashed. So the Messenger of Allah ﷺ told him to go back and perfect his ablution. Abu Dawood said: "This hadith is not well-known and it has not been narrated from Jarir ibn Hazim except by Ibn Wahb, and with this chain it means the same as the hadith of Qatadah, and this (narration) is Mursal." Abu Dawood said that Musa ibn Ismail narrated to him from Hammad, from Yunus and Humaid, from Al Hasan, from the Prophet ﷺ with a similar meaning to the hadith of Qatadah, and this is a Mursal hadith. Abu Dawood added.  

أَنَّ رَجُلًا جَاءَ إِلَى النَّبِيِّ ﷺ قَدْ تَوَضَّأَ وَتَرَكَ عَلَى قَدَمِهِ مِثْلَ مَوْضِعِ الظُّفُرِ فَقَالَ لَهُ رَسُولُ اللهِ ﷺ ارْجِعْ فَأَحْسِنْ وُضُوءَكَ قَالَ أَبُو دَاودَ وَلَيْسَ هَذَا الْحَدِيثُ بِمَعْرُوفٍ وَلَمْ يَرْوِهِ عَنْ جَرِيرِ بْنِ حَازِمٍ إِلَّا ابْنُ وَهْبٍ يَعْنِي بِهَذَا الْإِسْنَادِ قَالَ أَبُو دَاودَ ثنا مُوسَى بْنُ إِسْمَاعِيلَ ثنا حَمَّادٌ ثنا يُونُسُ وَحُمَيْدٌ عَنِ الْحَسَنِ عَنِ النَّبِيِّ ﷺ بِمَعْنَى حَدِيثِ قَتَادَةَ وَهَذَا مُرْسَلٌ قَالَ أَبُو دَاودَ  

[Machine] Narrated from the Prophet, ﷺ , that he said, "Go back and perfect your ablution." So he went back and then prayed. The sheikh informed us, Abu Nasr ibn Qatadah narrated to us, Abu al-Hasan Muhammad ibn Ahmad ibn Zakariya informed us, Abu al-Fadl Ahmad ibn Salamah al-Bazzar informed us, and Al-Hasan ibn Muhammad ibn A'yan informed us, Maqil mentioned its wording.  

عَنِ النَّبِيِّ ﷺ نَحْوَهُ قَالَ ارْجِعْ فَأَحْسِنْ وُضُوءَكَ فَرَجَعَ ثُمَّ صَلَّى قَالَ الشَّيْخُ أَخْبَرَنَاهُ أَبُو نَصْرِ بْنُ قَتَادَةَ ثنا أَبُو الْحَسَنِ مُحَمَّدُ بْنُ أَحْمَدَ بْنِ زَكَرِيَّا نا أَبُو الْفَضْلِ أَحْمَدُ بْنُ سَلَمَةَ الْبَزَّارُ نا الْحَسَنُ بْنُ مُحَمَّدِ بْنِ أَعْيَنَ نا مَعْقِلٌ فَذَكَرَ بِنَحْوِهِ  

[Machine] "That his command for ablution was within the realm of recommendation and indeed, the obligatory act is only the washing of that shining spot."  

أَنَّ أَمْرَهُ بِالْوضُوءِ كَانَ عَلَى طَرِيقِ الِاسْتِحْبَابِ وَإِنَّمَا الْوَاجِبُ غَسْلُ تِلْكَ اللُّمْعَةِ فَقَطْ  

[Machine] O Commander of the Faithful, the cold is intense and I do not have anything to warm me. He made him upset and then said to him, "Wash what you left from your feet and repeat the prayer." And he ordered him a cloak.  

يَا أَمِيرَ الْمُؤْمِنِينَ الْبَرْدُ شَدِيدٌ وَمَا مَعِيَ مَا يُدْفِئُنِي فَرَقَّ لَهُ بَعْدَمَا هَمَّ بِهِ فَقَالَ لَهُ اغْسِلْ مَا تَرَكْتَ مِنْ قَدَمِكَ وَأَعِدِ الصَّلَاةَ وَأَمَرَ لَهُ بِخَمِيصَةٍ  

[Machine] Narrated from Qutaibah with this wording, "And Ata' used to say that there is no harm in dividing the ablution into parts," and this is the saying of Al-Hasan and Al-Nakha'i, and the most correct saying of Al-Shafi'i.  

عَنْ قُتَيْبَةَ بِهَذَا اللَّفْظِ وَكَانَ عَطَاءٌ لَا يَرَى بِتَفْرِيقِ الْوُضُوءِ بَأْسًا وَهُوَ قَوْلُ الْحَسَنِ وَالنَّخَعِيِّ وَأَصَحُّ قُولَيِ الشَّافِعِيِّ ؓ  

[Machine] The Messenger of Allah ﷺ saw a group of people sitting on their heels, with the water not reaching the place of a dirham or the size of a fingernail. He said, "Woe to the heels from the Fire." And one of them was seen repeatedly performing ablution when he saw a spot on his heel that was not washed. This, if authenticated, is something they chose for themselves, and it is possible that he meant only to redo the ablution for that specific spot.  

رَأَى رَسُولُ اللهِ ﷺ قَوْمًا عَلَى أَعْقَابِ أَحَدِهِمْ مِثْلُ مَوْضِعِ الدِّرْهَمِ أَوْ مِثْلُ مَوْضِعِ الظُّفُرِ لَمْ يُصِبْهُ الْمَاءُ قَالَ فَجَعَلَ يَقُولُ وَيْلٌ لِلْأَعْقَابِ مِنَ النَّارِ قَالَ وَكَانَ أَحَدُهُمْ يَنْظُرُ أَنَّهُ إِذَا رَأَى بِعَقِبِهِ مَوْضِعًا لَمْ يصِبْهُ الْمَاءُ أَعَادَ وُضُوءَهُ وَهَذَا إِنْ صَحَّ فَشَيْءٌ اخْتَارُوهُ لِأَنْفُسِهِمْ وَقَدْ يُحْتَمَلُ أَنْ يرِيدَ بِهِ إِعَادَةَ وُضُوءِ ذَلِكَ الْمَوْضِعِ فَقَطْ