Allah said:

﴿فَمَن كَانَ مِنكُم مَّرِيضًا أَوْ بِهِ أَذًى مِّن رَّأْسِهِ فَفِدْيَةٌ مِّن صِيَامٍ أَوْ صَدَقَةٍ أَوْ نُسُكٍ﴾

(And whosoever of you is ill or has an ailment on his scalp (necessitating shaving), he must pay a Fidyah.)

Al-Bukhari reported that `Abdur-Rahman bin Asbahani said that he heard `Abdullah bin Ma`qil saying that he sat with Ka`b bin `Ujrah in the mosque of Kufah (in Iraq). He then asked him about the Fidyah of the fasting. Ka`b said, "This was revealed concerning my case especially, but it is also for you in general. I was carried to Allah's Messenger and the lice were falling in great numbers on my face. The Prophet said:

«مَا كُنْتُ أُرَى أَنَّ الْجَهْدَ بلَغَ بكَ هذَا، أمَا تَجِدُ شَاة»

؟ قُلْتُ: لَا

(I never thought that your ailment (or struggle) had reached to such an extent as I see. Can you afford a sheep (for sacrifice)' I replied in the negative.) He then said:

«صُمْ ثَلَاثَةَ أَيَّامٍ أَوْ أَطْعِمْ سِتَّةَ مَسَاكِينَ، لِكُلِّ مِسْكِينٍ نِصْفُ صَاعٍ مِنْ طَعَامٍ، وَاحْلِقْ رَأْسَك»

(Fast for three days or feed six poor persons, each with half a Sa` of food (1 Sa` = 3 kilograms approx.) and shave your head.)

So this is a general judgement derived from a specific case.

Imam Ahmad recorded that Ka`b bin `Ujrah said, "Allah's Messenger came by while I was igniting the fire under a pot and while the lice were falling down my head or my eyelids. He said:

«يُؤْذِيكَ هَوَامُّ رَأْسِك»

؟ قُلْتُ: نَعَمْ

(Do these lice in your head bother you) I said, `Yes.' He said:

«فَاحْلِقْهُ، وَصُمْ ثَلَاثَةَ أَيَّامٍ، أَوْ أَطْعِمْ سِتَّةَ مَسَاكِينَ، أَوِ انْسُكْ نَسِيكَة»

(Shave it, then fast three days, or feed six poor people, or sacrifice an animal.)

Ayyub (one of the narrators of the Hadith) commented, "I do not know which alternative was stated first.'' The wording of the Qur'an begins with the easiest then the more difficult options: "Pay a Fidyah of fasting (three days), feeding (six poor persons) or sacrificing (an animal).'' Meanwhile, the Prophet advised Ka`b with the more rewarding option first, that is, sacrificing a sheep, then feeding six poor persons, then fasting three days. Each Text is suitable in its place and context, all the thanks and praises are due to Allah.