(99) باب الْغُسْلِ مِنَ الْجَنَابَةِ
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.
Reference: Sunan Abi Dawud 249
In-book reference: Book 1, Hadith 249
English translation: Book 1, Hadith 249