Sahih Bukhari Volume 2, Book 20, Hadith Number 204.

Narated By Anas bin Sirin : We went to receive Anas bin Malik when he returned from Sham and met him at a place called ‘Ain-at-Tamr. I saw him praying riding the donkey, with his face to this direction, i.e. to the left of the Qibla. I said to him, “I have seen you offering the prayer in a direction other than that of the Qibla.” He replied, “If I had not seen Allah’s Apostle doing it, I would not have done it.”