Hadith: 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.

عن عبد الله بن مَعْقِلٍ قال: «جلستُ إلى كَعْبِ بن عُجْرَةَ، فسألته عن الفدية، فقال: نزلت فِيَّ خاصة. وهي لكم عامة. حُمِلْتُ إلى رسول الله صلى الله عليه وسلم وَالقَمْلُ يَتَنَاثَرُ على وجهي. فقال: ما كُنْتُ أُرَى الوَجَعَ بَلَغَ بِكَ ما أَرَى -أو ما كنت أُرَى الجَهْدَ بلغ بك ما أَرى-! أَتَجِدُ شاة؟ فقلت: لا. فقال: صم ثلاثة أيام، أو أطعم ستة مساكين -لكل مسكين نصف صاع-». وفي رواية: «فأمره رسول الله صلى الله عليه وسلم أن يُطْعِمَ فَرَقًا بَيْنَ سِتَّةِ ، أو يُهْدِي شَاةً ، أو يصوم ثلاثة أيام».
[صحيح] - [متفق عليه]

‘Abdullāh ibn Ma‘qil reported: I sat with Ka‘b ibn ‘Ujrah and asked him about the ransom and he replied: "It (the verse) was revealed concerning my case specifically, but it is also meant for you in general. 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.'" In another narration: "The Messenger of Allah (may Allah's peace and blessings be upon him) ordered him to either feed six people three Sā‘ (each will take half a Sā‘), slaughter a sheep as a sacrifice, or fast three days."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

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.