From Sahih Muslim (Hadith 2877):
Ka'b b. 'Ujra (Allah be pleased with him) reported:The Messenger of Allah (ﷺ) came to me on the occasion of Hudaibiya and I was kindling fire under my cooking pot and lice were creeping on my face. Thereupon he (the Holy Prophet) said: Do the vermins harm your head? I said: Yes. He said: Get your head shaved and (in lieu of it) observe fasts for three days or feed six needy persons, or offer sacrifice (of an animal). Ayyub said: I do not know with what (type of expiation) did he commence (the statement)
Reference: Muslim 2877