Sunan Abi Dawud 249
Narrated Ali ibn AbuTalib:
The Messenger of Allah (ﷺ) said: If anyone who is sexual defiled leaves a spot equal to the breadth of a hair without washing, such and such an amount of Hell-fire will have to be suffered for it. Ali said:  On that account I treated my head (hair) as an enemy, meaning I cut my hair. He used to cut the hair (of his head). May Allah be pleased with him.
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا حَمَّادٌ، أَخْبَرَنَا عَطَاءُ بْنُ السَّائِبِ، عَنْ زَاذَانَ، عَنْ عَلِيٍّ، - رضى الله عنه - أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏
"‏ مَنْ تَرَكَ مَوْضِعَ شَعْرَةٍ مِنْ جَنَابَةٍ لَمْ يَغْسِلْهَا فُعِلَ بِهِ كَذَا وَكَذَا مِنَ النَّارِ ‏"‏ ‏.‏ قَالَ عَلِيٌّ فَمِنْ ثَمَّ عَادَيْتُ رَأْسِي فَمِنْ ثَمَّ عَادَيْتُ رَأْسِي ثَلاَثًا ‏.‏ وَكَانَ يَجِزُّ شَعْرَهُ ‏.‏