Language:
كتاب الحج
25  
Hajj (Pilgrimage)
(127)
Chapter: To shave the head and to have the head-hair cut short on finishing the Ihram
(127)
باب الْحَلْقِ وَالتَّقْصِيرِ عِنْدَ الإِحْلاَلِ
Sahih al-Bukhari 1729

Narrated `Abdullah:

The Prophet (ﷺ) and some of his companions got their heads shaved and some others got their hair cut short.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ أَسْمَاءَ، حَدَّثَنَا جُوَيْرِيَةُ بْنُ أَسْمَاءَ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ، قَالَ حَلَقَ النَّبِيُّ صلى الله عليه وسلم وَطَائِفَةٌ مِنْ أَصْحَابِهِ، وَقَصَّرَ بَعْضُهُمْ‏.‏
Reference : Sahih al-Bukhari 1729
In-book reference : Book 25, Hadith 207
USC-MSA web (English) reference : Vol. 2, Book 26, Hadith 787
  (deprecated numbering scheme)