Sahih Bukhari | Volume 5 | Hadith Number 478

Narrated by Kab bin Ujra
That Allah's Apostle saw him with the lice falling (from his head) on his face. Allah's Apostle said, "Are your lice troubling you? Ka'b said, "Yes." Allah's Apostle thus ordered him to shave his head while he was at Al-Hudaibiya. Up to then there was no indication that all of them would finish their state of Ihram and they hoped that they would enter Mecca. Then the order of Al-Fidya was revealed, so Allah's Apostle ordered Kab to feed six poor persons with one Faraq of food or slaughter a sheep or fast for three days.