I was carried to the Messenger of Allah (may Allah's peace and blessings be upon him) and lice were falling on my face. He said: I never thought your ailment has reached such an extent. Can you afford a sheep (as a ransom for shaving your head)? I replied: No. He then said: Fast three days, or feed six poor people.

The Prophet (may Allah's peace and blessings be upon him) saw Ka‘b ibn ‘Ujrah in Al-Hudaybiyah while being in the state of Ihrām, and lice were falling down his face out of his illness. The Prophet (may Allah's peace and blessings be upon him) felt sorry for him and said he never thought his ailment was that serious. Then he asked Ka‘b: "Can you afford a sheep?" He said: 'No.' So, Allah, Blessed and Exalted, revealed the verse (which means): {And whoever among you is ill or has an ailment of the head [making shaving necessary] must offer a ransom of fasting [three days] or charity or sacrifice.} [Sūrat al-Baqarah: 196]
Thereupon, the Prophet (may Allah's peace and blessings be upon him) gave him choice either to fast three days, or feed six poor people; each receiving half a Sā‘ of wheat or the like. Both choices are an expiation for shaving his head during the state of Ihrām because he was forced due to the ailment in his head. In the other narration, the Prophet (may Allah's peace and blessings be upon him) gave him three options.