كتاب الحج
15  
The Book of Pilgrimage
(10)
Chapter: It is permissible for a Muhrim to shave his head if there is a problem, but it is obligatory to offer a Fidyah for shaving it, and Clarifying what the Fidyah is
(10)
باب جَوَازِ حَلْقِ الرَّأْسِ لِلْمُحْرِمِ إِذَا كَانَ بِهِ أَذًى وَوُجُوبِ الْفِدْيَةِ لِحَلْقِهِ وَبَيَانِ قَدْرِهَا
Sahih Muslim 1201 f

Ka'b b. Ujra (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) happened to pass by him during the period of Hudaibiya. Thereupon he (the Holy Prophet) said to him (Ka'b b. Ujra):

Do these vermins trouble your head? He said: Yes. Thereupon he (the Holy Prophet) said: Shave your head. Then sacrifice a goat or observe fasts for three days or give three sits of dates to feed six needy persons.
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا خَالِدُ بْنُ عَبْدِ اللَّهِ، عَنْ خَالِدٍ، عَنْ أَبِي قِلاَبَةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي لَيْلَى، عَنْ كَعْبِ بْنِ عُجْرَةَ، رضى الله عنه أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم مَرَّ بِهِ زَمَنَ الْحُدَيْبِيَةِ فَقَالَ لَهُ ‏"‏ آذَاكَ هَوَامُّ رَأْسِكَ ‏"‏ ‏.‏ قَالَ نَعَمْ ‏.‏ فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم ‏"‏ احْلِقْ رَأْسَكَ ثُمَّ اذْبَحْ شَاةً نُسُكًا أَوْ صُمْ ثَلاَثَةَ أَيَّامٍ أَوْ أَطْعِمْ ثَلاَثَةَ آصُعٍ مِنْ تَمْرٍ عَلَى سِتَّةِ مَسَاكِينَ ‏"‏ ‏.‏
Reference : Sahih Muslim 1201f
In-book reference : Book 15, Hadith 92
USC-MSA web (English) reference : Book 7, Hadith 2737
  (deprecated numbering scheme)