Allah said:

﴿ظپظژظ…ظژظ† ظƒظژط§ظ†ظژ ظ…ظگظ†ظƒظڈظ… ظ…ظ‘ظژط±ظگظٹط¶ظ‹ط§ ط£ظژظˆظ’ ط¨ظگظ‡ظگ ط£ظژط°ظ‹ظ‰ ظ…ظ‘ظگظ† ط±ظ‘ظژط£ظ’ط³ظگظ‡ظگ ظپظژظپظگط¯ظ’ظٹظژط©ظŒ ظ…ظ‘ظگظ† طµظگظٹظژط§ظ…ظچ ط£ظژظˆظ’ طµظژط¯ظژظ‚ظژط©ظچ ط£ظژظˆظ’ ظ†ظڈط³ظڈظƒظچ﴾

(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:

آ«ظ…ظژط§ ظƒظڈظ†ظ’طھظڈ ط£ظڈط±ظژظ‰ ط£ظژظ†ظ‘ظژ ط§ظ„ظ’ط¬ظژظ‡ظ’ط¯ظژ ط¨ظ„ظژط؛ظژ ط¨ظƒظژ ظ‡ط°ظژط§طŒ ط£ظ…ظژط§ طھظژط¬ظگط¯ظڈ ط´ظژط§ط©آ»

طں ظ‚ظڈظ„ظ’طھظڈ: ظ„ظژط§

(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:

آ«طµظڈظ…ظ’ ط«ظژظ„ظژط§ط«ظژط©ظژ ط£ظژظٹظ‘ظژط§ظ…ظچ ط£ظژظˆظ’ ط£ظژط·ظ’ط¹ظگظ…ظ’ ط³ظگطھظ‘ظژط©ظژ ظ…ظژط³ظژط§ظƒظگظٹظ†ظژطŒ ظ„ظگظƒظڈظ„ظ‘ظگ ظ…ظگط³ظ’ظƒظگظٹظ†ظچ ظ†ظگطµظ’ظپظڈ طµظژط§ط¹ظچ ظ…ظگظ†ظ’ ط·ظژط¹ظژط§ظ…ظچطŒ ظˆظژط§ط­ظ’ظ„ظگظ‚ظ’ ط±ظژط£ظ’ط³ظژظƒآ»

(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:

آ«ظٹظڈط¤ظ’ط°ظگظٹظƒظژ ظ‡ظژظˆظژط§ظ…ظ‘ظڈ ط±ظژط£ظ’ط³ظگظƒآ»

طں ظ‚ظڈظ„ظ’طھظڈ: ظ†ظژط¹ظژظ…ظ’

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

آ«ظپظژط§ط­ظ’ظ„ظگظ‚ظ’ظ‡ظڈطŒ ظˆظژطµظڈظ…ظ’ ط«ظژظ„ظژط§ط«ظژط©ظژ ط£ظژظٹظ‘ظژط§ظ…ظچطŒ ط£ظژظˆظ’ ط£ظژط·ظ’ط¹ظگظ…ظ’ ط³ظگطھظ‘ظژط©ظژ ظ…ظژط³ظژط§ظƒظگظٹظ†ظژطŒ ط£ظژظˆظگ ط§ظ†ظ’ط³ظڈظƒظ’ ظ†ظژط³ظگظٹظƒظژط©آ»

(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.