This hadeeth comes under the following chapter: PERMISSIBILITY OF SAYING NAFL PRAYER ON A RIDING BEAST WHILE ON A JOURNEY, IN WHATEVER DIRECTION IT TURNS
Ibn 'Umar reported that the Messenger of Allaah (sallAllaahu alayhi wa sallam) 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 Allaah's face" (ii. 115).