The Messenger of Allah (ﷺ) said: There is sexual defilement under every hair; so wash the hair and cleanse the skin.

Abu Dawud said: The tradition narrated by Harith b. Wajih is rejected (Munkar). He is weak (in transmission).
Sunan Abi Dawud Book 1, Hadith 248
← Previous Hadith There were fifty prayers (obligatory in the beginning); and (in the beginning of Islam) washing seven times because of sexual defilement (was obligatory); and washing the urine from the cloth seven times (was obligatory).

The Apostle of Allah (ﷺ) kept on praying to Allah until the number of prayers was reduced to five and washing because of sexual defilement was allowed only once and washing the urine from the clothe was also permitted only once.

Sunan Abi Dawud Book 1, Hadith 247 Next Hadith → 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.

Sunan Abi Dawud Book 1, Hadith 249