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.
English reference: Vol. 1, Book 1, Hadith 599
Arabic reference: Book 1, Hadith 642