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.'"

حَدَّثَنَا حَرْمَلَةُ بْنُ يَحْيَى، حَدَّثَنَا عَبْدُ اللَّهِ بْنُ وَهْبٍ، حَدَّثَنَا جَرِيرُ بْنُ حَازِمٍ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ رَجُلاً، أَتَى النَّبِيَّ ـ ﷺ ـ وَقَدْ تَوَضَّأَ وَتَرَكَ مَوْضِعَ الظُّفْرِ لَمْ يُصِبْهُ الْمَاءُ فَقَالَ لَهُ النَّبِيُّ ـ ﷺ ـ " ارْجِعْ فَأَحْسِنْ وُضُوءَكَ " .