"The Messenger of Allah saw a man performing ablution and he missed a spot the size of a fingernail on his foot. He commanded him to repeat the ablution and his prayer, so he did."
رَأَى رَسُولُ اللَّهِ ﷺ رَجُلاً تَوَضَّأَ فَتَرَكَ مَوْضِعَ الظُّفْرِ عَلَى قَدَمِهِ فَأَمَرَهُ أَنْ يُعِيدَ الْوُضُوءَ وَالصَّلاَةَ قَالَ فَرَجَعَ
'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