Sahih Muslim 243
Sahih Muslim · Book 2: The Book of Purification · The obligation of completely washing all the parts to be washed when purifying oneself
Jabir reported:
حَدَّثَنِي سَلَمَةُ بْنُ شَبِيبٍ ، حَدَّثَنَا الْحَسَنُ بْنُ مُحَمَّدِ بْنِ أَعْيَنَ ، حَدَّثَنَا مَعْقِلٌ ، عَنْ أَبِي الزُّبَيْرِ ، عَنْ جَابِرٍ ، أَخْبَرَنِي عُمَرُ بْنُ الْخَطَّابِ ، أَنَّ رَجُلاً، تَوَضَّأَ فَتَرَكَ مَوْضِعَ ظُفُرٍ عَلَى قَدَمِهِ فَأَبْصَرَهُ النَّبِيُّ صلى الله عليه وسلم فَقَالَ " ارْجِعْ فَأَحْسِنْ وُضُوءَكَ " . فَرَجَعَ ثُمَّ صَلَّى .
'Umar b. Khattab said that a person performed ablution and left a small part equal to the space of a nail (unwashed). The Apostle of Allah (ﷺ) saw that and said: Go back and perform ablution well. He then went back (performed ablution well) and offered the prayer.