Musnad Ahmad Hadith 18128

It is narrated from Ka'b bin Ujrah (may Allah be pleased with him) that we were in the state of Ihram at Hudaybiyyah with the Noble Prophet (ﷺ). The polytheists had surrounded us (not allowing us to enter the Haram). My hair had grown very long, and during this time, lice began to fall from my head onto my face. The Noble Prophet (ﷺ) passed by me and said, "Are the lice troubling you?" I replied, "Yes!" The Noble Prophet (ﷺ) ordered, "Shave your head." On this occasion, this verse was revealed: "And whoever among you is ill or has an ailment of the head (necessitating shaving), he should offer a ransom by fasting, or charity, or sacrifice." So the Noble Prophet (ﷺ) ordered me, "Fast for three days, or give charity of half a sa' of food to six needy people, or offer a sacrifice—whichever is easiest for you."