Sahih Al Bukhari Hadith Number 1104

Narrated `Abdullah bin Amir that his father had told him that he had seen the Prophet (peace be upon him) praying Nawafil at night on the back of his Mount on a journey, facing whatever direction it took.

18  Shortening the Prayers (At-Taqseer)
Hadith number 1004
Volume no 2
Chapter: 12 (Whoever offered Nawafil not after the compulsory Salat but before it)