Sahih Bukhari Volume 3, Book 27, Hadith 1817

حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شُعْبَةُ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ الأَصْبِهَانِيِّ، عَنْ عَبْدِ اللَّهِ بْنِ مَعْقِلٍ، قَالَ جَلَسْتُ إِلَى كَعْبِ بْنِ عُجْرَةَ ـ رضى الله عنه ـ فَسَأَلْتُهُ عَنِ الْفِدْيَةِ، فَقَالَ نَزَلَتْ فِيَّ خَاصَّةً، وَهْىَ لَكُمْ عَامَّةً، حُمِلْتُ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم وَالْقَمْلُ يَتَنَاثَرُ عَلَى وَجْهِي فَقَالَ ‏"‏ مَا كُنْتُ أُرَى الْوَجَعَ بَلَغَ بِكَ مَا أَرَى أَوْ مَا كُنْتُ أُرَى الْجَهْدَ بَلَغَ بِكَ مَا أَرَى، تَجِدُ شَاةً ‏"‏‏.‏ فَقُلْتُ لاَ‏.‏ فَقَالَ ‏"‏ فَصُمْ ثَلاَثَةَ أَيَّامٍ، أَوْ أَطْعِمْ سِتَّةَ مَسَاكِينَ، لِكُلِّ مِسْكِينٍ نِصْفَ صَاعٍ ‏"

Narrated By 'Abdullah bin Ma'qal : I sat with Ka'b 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.)