Sahih Bukhari, Chapter: Pilgrims Prevented from Completing the Pilgrimage

I sat with Kab bin Ujra and asked him about the Fidya. He replied, "This revelation was revealed concerning my case especially, but it is also for you in general. I was carried to Allah's Apostle and the lice were falling in great number on my face. The Prophet said, "I have never thought that your ailment (or struggle) has reached to such an extent as I see. Can you afford a sheep?" I replied in the negative. He then said, "Fast for three days, or feed six poor persons each with half a Sa of food." (1 Sa = 3 Kilograms approx.)