From Sahih Bukhari (Hadith 5703):
Narrated Ka`b bin Ujrah:The Prophet (ﷺ) 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:
Reference: Bukhari 5703