Hadith: A man performed ablution and left a small part on his foot equal to the size of a nail unwashed. The Prophet (may Allah's peace and blessings be upon him) saw that and said: "Go back and perform your ablution properly." He performed it again and prayed

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] - [صحيح مسلم: 243]

Jābir (may Allah be pleased with him) reported: ‘Umar ibn al-Khattāb said to me:
A man performed ablution and left a small part on his foot equal to the size of a nail unwashed. The Prophet (may Allah's peace and blessings be upon him) saw that and said: "Go back and perform your ablution properly." He performed it again and prayed.
[Authentic for being narrated by another companion] - [Narrated by Muslim] - [Sahih Muslim - 243]

Explanation

‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) saw a man who had just finished performing ablution leaving a spot on his foot, the size of a fingernail, unwashed. He said to him while pointing to the place that was not properly washed: Go back and perform your ablution properly and perfectly and wash each organ properly with water. So, the man went back, performed ablution perfectly, and then prayed.

Benefits from the Hadith

1. Hastening to enjoin good and guide the ignorant and the heedless is obligatory, especially if the evil involved results in invalidating his act of worship.
2. It is obligatory to thoroughly wash the body parts required in ablution and whoever leaves out any spot - even a tiny one - his ablution will be invalid and he is required to repeat it if such a spot is noticed after some time.
3. It is permissible to perfect ablution by performing it completely in the manner prescribed by the Shariah.
4. The feet are from the body parts required in ablution and wiping them is not sufficient; rather, they must be washed.
5. The prescribed succession between the body parts required in ablution should be observed in such a way that each part is washed before the one preceding it becomes dry.
6. Ignorance and forgetfulness do not make an obligation cease to be due but they absolve the sin. This man who did not perform ablution perfectly due to his ignorance, the Prophet (may Allah's peace and blessings be upon him) did not exempt him from the obligation, namely ablution; rather, he ordered him to repeat it.