Narrated Ka'b bin Ujra:

We were in the company of Allah's Apostle at Al-Hudaibiya in the state of Ihram and the pagans did not allow us to proceed (to the Ka'ba). I had thick hair and lice started falling on my face. The Prophet ﷺ passed by me and said, "Are the lice of your head troubling you?" I replied, Yes." (The sub-narrator added, "Then the following Divine Verse was revealed:-- "And if anyone of you is ill or has an ailment in his scalp, (necessitating shaving) must pay a ransom (Fida) of either fasting or feeding the poor, Or offering a sacrifice." (2.196)

حَدَّثَنِي مُحَمَّدُ بْنُ هِشَامٍ أَبُو عَبْدِ اللَّهِ، حَدَّثَنَا هُشَيْمٌ، عَنْ أَبِي بِشْرٍ، عَنْ مُجَاهِدٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي لَيْلَى، عَنْ كَعْبِ بْنِ عُجْرَةَ، قَالَ كُنَّا مَعَ رَسُولِ اللَّهِ ﷺ بِالْحُدَيْبِيَةِ وَنَحْنُ مُحْرِمُونَ، وَقَدْ حَصَرَنَا الْمُشْرِكُونَ ـ قَالَ ـ وَكَانَتْ لِي وَفْرَةٌ فَجَعَلَتِ الْهَوَامُّ تَسَّاقَطُ عَلَى وَجْهِي، فَمَرَّ بِي النَّبِيُّ ﷺ فَقَالَ " أَيُؤْذِيكَ هَوَامُّ رَأْسِكَ ". قُلْتُ نَعَمْ. قَالَ وَأُنْزِلَتْ هَذِهِ الآيَةُ {فَمَنْ كَانَ مِنْكُمْ مَرِيضًا أَوْ بِهِ أَذًى مِنْ رَأْسِهِ فَفِدْيَةٌ مِنْ صِيَامٍ أَوْ صَدَقَةٍ أَوْ نُسُكٍ}