Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 503
Narrated Ka'b bin Ujra:
We were in the company of Allah's Apostle at Al-Hudaibiya in the state of Ihram and the pagans did not allow us to proceed (to the Ka'ba). I had thick hair and lice started falling on my face. The Prophet passed by me and said, "Are the lice of your head troubling you?" I replied, Yes." (The sub-narrator added, "Then the following Divine Verse was revealed:-- "And if anyone of you is ill or has an ailment in his scalp, (necessitating shaving) must pay a ransom (Fida) of either fasting or feeding the poor, Or offering a sacrifice." (2.196)