Sahih Muslim 1201 d
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 (may peace be, upon him) stood near him and lice were falling from his head. Thereupon he (the Prophet) said: Do these vermins trouble you? I said: Yes. Thereupon he said: Then shave your head; and it was in connection with me that this verse was revealed:" Whoever among you is sick or has an ailment of the head, he (may effect) a compensation by fasting or alms or a sacrifice". He (the Prophet, therefore) said to me: Observe fast for three days or give a quantity of alms enough to feed six needy persons or offer sacrifice (of an animal) that is available.