Hadith: Go back and perform ablution properly

عَنْ جَابِرٍ رضي الله عنه قال: أَخْبَرَنِي عُمَرُ بْنُ الْخَطَّابِ:

أَنَّ رَجُلًا تَوَضَّأَ فَتَرَكَ مَوْضِعَ ظُفُرٍ عَلَى قَدَمِهِ فَأَبْصَرَهُ النَّبِيُّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ فَقَالَ: «ارْجِعْ فَأَحْسِنْ وُضُوءَكَ» فَرَجَعَ، ثُمَّ صَلَّى.
[صحيح بشواهده] - [رواه مسلم]

‘Umar ibn al-Khattāb (may Allah be pleased with him) reported: A man performed ablution and left a small part on his foot equal to the space of a nail unwashed. The Messenger of Allah (may Allah’s peace and blessings be upon him) saw that and said: "Go back and perform ablution properly." He performed it again and prayed.
Sahih/Authentic. - [Muslim]

Explanation

‘Umar ibn al-Khattāb (may Allah be pleased with him) said that a man performed ablution, but did not do it properly. He left a spot on his foot, the size of a fingernail, unwashed. He missed that place and the water did not reach it. Having seen this, the Prophet (may Allah's peace and blessings be upon him) told the man to go back and perform ablution again in the manner prescribed by the Shariah, not missing any place that should be washed. So the man went back, performed ablution afresh, and prayed.