From Sahih Bukhari (Hadith 1817):
Narrated `Abdur-Rahman bin Abu Layla:(Reporting the speech of Ka`b bin Ujra) Allah's Messenger (ﷺ) saw him (i.e. Ka`b) while the lice were falling on his face. He asked (him), "Have your lice troubled you?" He replied in the affirmative. So, he ordered him to get his head shaved while he was at Al-Hudaibiya. At that time they were not permitted to finish their Ihram, and were still hoping to enter Mecca. So, Allah revealed the verses of Al-Fidya. Allah's Messenger (ﷺ) ordered him to feed six poor persons with one Faraq of food or to slaughter one sheep (as a sacrifice) or to fast for three days
Reference: Bukhari 1817