Sahih Muslim 1201 e
The Book of Pilgrimage
It is permissible for a Muhrim to shave his head if there is a problem, but it is obligatory to offer a Fidyah for shaving it, and Clarifying what the Fidyah is
Ka`b b. 'Ujra (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) happened to pass by him at Hudaibiya before entering Mecca in a state of Ihram and he (Ka'b) was kindling fire under the cooking pot and vermin were creeping on his (Ka`b's) face. Thereupon (the Prophet) said: Do these vermin trouble you? He (Ka'b) said: Yes. The Messenger of Allah (ﷺ) said: Shave your head and give some quantity of food enough to feed six needy persons (faraq is equal to three sa's), or observe fast for three days or offer sacrifice of a sacrificial animal. Ibn Najih (one of the narrators) said:" Or sacrifice a goat."