Sahih Al Bukhari Hadith Number 1099

Narrated Jabir bin `Abdullah: The Prophet (peace be upon him used to pray (the Nawafil) on his Mount facing east and whenever he wanted to offer the compulsory prayer, he used to dismount and face the Qibla.

18  Shortening the Prayers (At-Taqseer)
Hadith number 1099
Volume no 2
Chapter: 9 (To get down to offer compulsory Salat)