Narrated '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 ﷺ 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.)

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 ﷺ 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 ".