Kaʿb bin 'Ujra said that Messenger of Allah ﷺ said to him (Kaʿb), "Perhaps your lice have troubled you?" Kaʿb replied, "Yes! O Messenger of Allah ﷺ." Messenger of Allah ﷺ said, "Have your head shaved and then either fast three days or feed six poor persons or slaughter one sheep as a sacrifice."  

عَنْ رَسُولِ اللَّهِ ﷺ أَنَّهُ قَالَ لَعَلَّكَ آذَاكَ هَوَامُّكَ قَالَ نَعَمْ يَا رَسُولَ اللَّهِ فَقَالَ رَسُولُ اللَّهِ ﷺ احْلِقْ رَأْسَكَ وَصُمْ ثَلاَثَةَ أَيَّامٍ أَوْ أَطْعِمْ سِتَّةَ مَسَاكِينَ أَوِ انْسُكْ بِشَاةٍ  

I sat with Kaʿb bin Ujra in this mosque, i.e. Kufa Mosque, and asked him about the meaning of: "Pay a ransom (i.e. Fidya) of either fasting or . . . . (2.196)" He said, "I was taken to the Prophet ﷺ while lice were falling on my face. The Prophet ﷺ said, 'I did not think that your trouble reached to such an extent. Can you afford to slaughter a sheep (as a ransom for shaving your head)?' I said, 'No.' He said, 'Then fast for three days, or feed six poor persons by giving half a Sa of food for each and shave your head.' So the above Verse was revealed especially for me and generally for all of you."  

حُمِلْتُ إِلَى النَّبِيِّ ﷺ وَالْقَمْلُ يَتَنَاثَرُ عَلَى وَجْهِي فَقَالَ مَا كُنْتُ أُرَى أَنَّ الْجَهْدَ قَدْ بَلَغَ بِكَ هَذَا أَمَا تَجِدُ شَاةً قُلْتُ لاَ قَالَ صُمْ ثَلاَثَةَ أَيَّامٍ أَوْ أَطْعِمْ سِتَّةَ مَسَاكِينَ لِكُلِّ مِسْكِينٍ نِصْفُ صَاعٍ مِنْ طَعَامٍ وَاحْلِقْ رَأْسَكَ فَنَزَلَتْ فِيَّ خَاصَّةً وَهْىَ لَكُمْ عَامَّةً