Sahih Muslim 1201 h
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 he went out with the Messenger of Allah (ﷺ) in the state of Ihram, and his (Ka'b's) head and beard were infested with lice. This was conveyed to the Messenger of Allah (ﷺ). He sent for him (Ka'b) and called a barber (who) shaved his head. He (the Prophet) said. Is there any sacrificial animal with you? He (Kalb) said: I cannot afford it. He then commanded him to observe fasts for three days or feed six needy persons, one sa' for every two needy persons. And Allah the Exalted and Majestic revealed this (verse) particular with regard to him:" So whosoever among you is sick and has an ailment of the head.." ; then (its application) became general for the Muslims.