Sunan Ibn Majah, Chapter: The Chapters on Hajj

"I sat with Kab bin Ujrah in the mosque and asked him about this Verse: 'He must pay a compensation of either fasting (three days) or giving charity (feeding six poor persons) or offering sacrifice (one sheep).' [2:196] Kab said: it was revealed concerning me. I had trouble with my head, so I was carried to the Messenger of Allah (saw), with lice crawling on my face. He said: 'I did not think that you were suffering as much as I see. Do you have a sheep?' I said: 'No.' Then this Verse was revealed: "He must pay a Fidyah (ransom) of either fasting (three days) or giving Sadaqah (charity - feeding six poor persons) or offering sacrifice (one sheep)." [2:196] He said: 'Fasting is three days, charity is to be given to six poor persons, giving each one half of a Sa' of food, and the sacrifice is a sheep.'"