'Umar b. Khattab said that a person performed ablution and left a small part equal to the space of a nail (unwashed). The Apostle of Allah ﷺ saw that and said: Go back and perform ablution well. He then went back (performed ablution well) and offered the prayer.  

أَنَّ رَجُلاً تَوَضَّأَ فَتَرَكَ مَوْضِعَ ظُفُرٍ عَلَى قَدَمِهِ فَأَبْصَرَهُ النَّبِيُّ ﷺ فَقَالَ ارْجِعْ فَأَحْسِنْ وُضُوءَكَ فَرَجَعَ ثُمَّ صَلَّى  

A man came to the Prophet; he had performed ablution and has missed a spot the size of a fingernail where water had not reached. The Prophet said to him: 'Go back and perform ablution properly.'"  

أَنَّ رَجُلاً أَتَى النَّبِيَّ ﷺ وَقَدْ تَوَضَّأَ وَتَرَكَ مَوْضِعَ الظُّفْرِ لَمْ يُصِبْهُ الْمَاءُ فَقَالَ لَهُ النَّبِيُّ ﷺ ارْجِعْ فَأَحْسِنْ وُضُوءَكَ  

A person came to the Messenger of Allah ﷺ. He performed ablution and left a small part equal to the space of a nail upon his foot. The Messenger of Allah ﷺ said to him : Go back and perform ablution well. Abu Dawud said: This tradition is not known through Jarir b. Hazim. It was transmitted only by Ibn Wahab. Another version adds the wording : “ Go back and perform the ablution well.” (Using translation from Abū Dāʾūd 173)   

أَنَّ رَجُلًا جَاءَ إِلَى النَّبِيِّ ﷺ قَدْ تَوَضَّأَ وَتَرَكَ عَلَى قَدَمِهِ مِثْلَ مَوْضِعِ الظُّفُرِ فَقَالَ لَهُ رَسُولُ اللهِ ﷺ ارْجِعْ فَأَحْسِنْ وُضُوءَكَ