Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1499
Ibn 'Umar reported that the Messenger of Allah (may peace be upon him) used to say prayer on his camel while coming from Mecca to Medina, in whatever direction his face had turned; and its was (in this context) that this verse was revealed:" So whether you turn thither is Allah's face" (ii. 115).