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  

"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