Language:
كتاب الحج
25  
Hajj (Pilgrimage)
(125)
Chapter: Slaughtering before having one’s head shaved
(125)
باب الذَّبْحِ قَبْلَ الْحَلْقِ
Sahih al-Bukhari 1723

Narrated Ibn `Abbas:

The Prophet (ﷺ) was asked by a man who said, "I have done the Rami in the evening." The Prophet (ﷺ) replied, "There is no harm in it." Another man asked, "I had my head shaved before the slaughtering." The Prophet (ﷺ) replied, "There is no harm in it."

حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا عَبْدُ الأَعْلَى، حَدَّثَنَا خَالِدٌ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ سُئِلَ النَّبِيُّ صلى الله عليه وسلم فَقَالَ رَمَيْتُ بَعْدَ مَا أَمْسَيْتُ‏.‏ فَقَالَ ‏"‏ لاَ حَرَجَ ‏"‏‏.‏ قَالَ حَلَقْتُ قَبْلَ أَنْ أَنْحَرَ‏.‏ قَالَ ‏"‏ لاَ حَرَجَ ‏"‏‏.‏
Reference : Sahih al-Bukhari 1723
In-book reference : Book 25, Hadith 201
USC-MSA web (English) reference : Vol. 2, Book 26, Hadith 781
  (deprecated numbering scheme)