Maliks Muwatta Book 20, Hadith Number 246.

Yahya related to me from Malik from Abd al-Karim ibn Malik al-Jazari from Abd ar-Rahman ibn Abi Layla from Kab ibn Ujra that one time he was with the Messenger of Allah, may Allah bless him and grant him peace, in ihram, and he was suffering from lice on his head. The Messenger of Allah, may Allah bless him and grant him peace, told him to shave his head, saying, “Fast three days, or feed six poor people, two mudds for each person, or sacrifice a sheep. If you do any of those it will be enough for you.”