The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1503
'Abdullah b. Dinar reported on the authority of Ibn 'Umar that the Messenger of Allah (may peace be upon him) used to observe prayer on his ride (no matter) in which direction it had its face turned. 'Abdullah b. Dinar said that Ibn 'Umar used to do like that.