From Sahih Bukhari (Hadith 1815):
Narrated Ka`b bin `Umra:Allah's Messenger (ﷺ) stood beside me at Al-Hudaibiya and the lice were falling from my head in great number. He asked me, "Have your lice troubled you?" I replied in the affirmative. He ordered me to get my head shaved. Ka`b added, "This Holy Verse:--'And if any of you is ill, or has ailment in his scalp (2.196), etc. was revealed regarding me. "The Prophet (ﷺ) then ordered me either to fast three days, or to feed six poor persons with one Faraq (three Sas) (of dates), or to slaughter a sheep, etc. (sacrifice) whatever was available
Reference: Bukhari 1815