Language:
كتاب التقصير
18  
Shortening the Prayers (At-Taqseer)
(12)
Chapter: Whoever offered Nawafil not after the compulsory Salat but before it
(12)
باب مَنْ تَطَوَّعَ فِي السَّفَرِ فِي غَيْرِ دُبُرِ الصَّلَوَاتِ وَقَبْلَهَا
Sahih al-Bukhari 1104
Narrated `Abdullah bin Amir that his father had told him that he had seen the Prophet (p.b.u.h) praying Nawafil at night on the back of his Mount on a journey, facing whatever direction it took.
وَقَالَ اللَّيْثُ حَدَّثَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ حَدَّثَنِي عَبْدُ اللَّهِ بْنُ عَامِرٍ، أَنَّ أَبَاهُ، أَخْبَرَهُ أَنَّهُ، رَأَى النَّبِيَّ صلى الله عليه وسلم صَلَّى السُّبْحَةَ بِاللَّيْلِ فِي السَّفَرِ عَلَى ظَهْرِ رَاحِلَتِهِ حَيْثُ تَوَجَّهَتْ بِهِ‏.‏
Reference : Sahih al-Bukhari 1104
In-book reference : Book 18, Hadith 24
USC-MSA web (English) reference : Vol. 2, Book 20, Hadith 207
  (deprecated numbering scheme)