ENOfficial
It was narrated from Anas that: 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 (ﷺ) said to him: 'Go back and perform ablution properly.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا حَرْمَلَةُ بْنُ يَحْيَى، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ وَهْبٍ، حَدَّثَنَا جَرِيرُ بْنُ حَازِمٍ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ رَجُلاً، أَتَى النَّبِيَّ ـ صلى الله عليه وسلم ـ وَقَدْ تَوَضَّأَ وَتَرَكَ مَوْضِعَ الظُّفْرِ لَمْ يُصِبْهُ الْمَاءُ فَقَالَ لَهُ النَّبِيُّ ـ صلى الله عليه وسلم ـ " ارْجِعْ فَأَحْسِنْ وُضُوءَكَ " .
sunnah.com