Sahih al-Bukhari 1815

Abu Nuaym narrated to us; Sayf narrated to us, saying: Mujahid told me that he heard Abd al-Rahman ibn Abi Layla say that Kaab ibn Ujrah told him that at al-Hudaybiyyah the Messenger of Allah stopped by him while lice were falling from his head. He asked whether the vermin were troubling him. Kaab replied that they were. The Prophet told him to shave his head, or, as the narrator reported, he said simply to shave. Kaab said that this verse was revealed concerning him: Whoever among you is ill or has an ailment of the head... through the end of the verse. The Prophet then said: Fast for three days, or give a faraq of food among six people, or offer whatever sacrifice is readily available.