Sahih Bukhari Volume 5, Book 64, Hadith 4161

حَدَّثَنَا الْحَسَنُ بْنُ خَلَفٍ، قَالَ حَدَّثَنَا إِسْحَاقُ بْنُ يُوسُفَ، عَنْ أَبِي بِشْرٍ، وَرْقَاءَ، عَنِ ابْنِ أَبِي نَجِيحٍ، عَنْ مُجَاهِدٍ، قَالَ حَدَّثَنِي عَبْدُ الرَّحْمَنِ بْنُ أَبِي لَيْلَى، عَنْ كَعْبِ بْنِ عُجْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم رَآهُ وَقَمْلُهُ يَسْقُطُ عَلَى وَجْهِهِ فَقَالَ ‏"‏ أَيُؤْذِيكَ هَوَامُّكَ ‏"‏‏.‏ قَالَ نَعَمْ‏.‏ فَأَمَرَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يَحْلِقَ وَهْوَ بِالْحُدَيْبِيَةِ، لَمْ يُبَيِّنْ لَهُمْ أَنَّهُمْ يَحِلُّونَ بِهَا، وَهُمْ عَلَى طَمَعٍ أَنْ يَدْخُلُوا مَكَّةَ، فَأَنْزَلَ اللَّهُ الْفِدْيَةَ، فَأَمَرَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يُطْعِمَ فَرَقًا بَيْنَ سِتَّةِ مَسَاكِينَ، أَوْ يُهْدِيَ شَاةً، أَوْ يَصُومَ ثَلاَثَةَ أَيَّامٍ‏

Narrated By Kab bin Ujra : That Allah's Apostle saw him with the lice falling (from his head) on his face. Allah's Apostle said, "Are your lice troubling you? Ka'b said, "Yes." Allah's Apostle thus ordered him to shave his head while he was at Al-Hudaibiya. Up to then there was no indication that all of them would finish their state of Ihram and they hoped that they would enter Mecca. Then the order of Al-Fidya was revealed, so Allah's Apostle ordered Kab to feed six poor persons with one Faraq of food or slaughter a sheep or fast for three days.