Sahih Bukhari Volume 8, Book 79, Hadith Number 699.

Narated By Ka’b bin ‘Ujra: I came to the Prophet and he said to me, “Come near.” So I went near to him and he said, “Are your lice troubling you?” I replied, “Yes.” He said, “(Shave your head and) make expiation in the form of fasting, Sadaqa (giving in charity), or offering a sacrifice.” (The sub-narrator) Aiyub said, “Fasting should be for three days, and the Nusuk (sacrifice) is to be a sheep, and the Sadaqa is to be given to six poor persons.”