Narrated Ka`b bin Ujra:

We were in the company of Allah's Messenger (ﷺ) 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 (ﷺ) 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)