Sahih Bukhari Volume 007, Book 071, Hadith Number 604.
Narrated By Ka'b bin Ujrah: The Prophet came to me during the period of Al-Hudaibiya, while I was lighting fire underneath a cooking pot and lice were falling down my head. He said, "Do your lice hurt your?" I said, "Yes." He said, "Shave your head and fast for three days or feed six poor persons or slaughter a sheep as a sacrifice."