Sahih Muslim Book 7, Hadith Number 2739.

Chapter : It is permissible to shave the head in the state of Ihram if there is a trouble, and expiation becomes obligatory for shaving.

Ka’b b. Ujra (Allah be pleased with him) reported that he went out with the Apostle of Allah (may peace be upon him) in the state of Ihram, and his (Ka’b’s) head and beard were infested with lice. This was conveyed to the Apostle of Allah (may peace be upon him). He sent for him (Ka’b) and called a barber (who) shaved his head. He (the Holy 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.