Sunan Ibn Majah, Book 1, Hadith 710 (In-Book)

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 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 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 ‏" ‏ ‏.‏

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