Language:
كتاب الحج
25  
Hajj (Pilgrimage)
(28)
Chapter: Reciting Talbiya when one has mounted his mount and it stood up straight (ready to set out)
(28)
باب مَنْ أَهَلَّ حِينَ اسْتَوَتْ بِهِ رَاحِلَتُهُ قائِمَة
Sahih al-Bukhari 1552

Narrated Ibn `Umar:

The Prophet (p.b.u.h) recited Talbiya when he had mounted his Mount and was ready to set out.

حَدَّثَنَا أَبُو عَاصِمٍ، أَخْبَرَنَا ابْنُ جُرَيْجٍ، قَالَ أَخْبَرَنِي صَالِحُ بْنُ كَيْسَانَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـ قَالَ أَهَلَّ النَّبِيُّ صلى الله عليه وسلم حِينَ اسْتَوَتْ بِهِ رَاحِلَتُهُ قَائِمَةً‏.‏
Reference : Sahih al-Bukhari 1552
In-book reference : Book 25, Hadith 38
USC-MSA web (English) reference : Vol. 2, Book 26, Hadith 624
  (deprecated numbering scheme)