It was narrated from 'Ali bin Abu Talib that:The Prophet said: "Whoever leaves an area the size of a hair on his body and does not cleanse it from sexual impurity, such and such will be done to him in the Fire." 'Ali said: "Because of that I am hostile towards my hair," and he used to shave his head.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا الأَسْوَدُ بْنُ عَامِرٍ، حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ عَطَاءِ بْنِ السَّائِبِ، عَنْ زَاذَانَ، عَنْ عَلِيِّ بْنِ أَبِي طَالِبٍ، عَنِ النَّبِيِّ ـ صلى الله عليه وسلم ـ قَالَ " مَنْ تَرَكَ مَوْضِعَ شَعَرَةٍ مِنْ جَسَدِهِ مِنْ جَنَابَةٍ لَمْ يَغْسِلْهَا فُعِلَ بِهِ كَذَا وَكَذَا مِنَ النَّارِ " . قَالَ عَلِيٌّ فَمِنْ ثَمَّ عَادَيْتُ شَعَرِي . وَكَانَ يَجُزُّهُ .
IbnMaja
The Book of Purification and its Sunnah • Hadith 599