(2) باب الْحَالِ الَّتِي يَجُوزُ عَلَيْهَا اسْتِقْبَالُ غَيْرِ الْقِبْلَةِ
It was narrated that Ibn Umar said:
"The messenger of Allah (peace be upon him) used to pray atop his mount while travelling, facing whatever direction it was facing." (One of the narrators) Malik said: "Abdullah bin Dinar said: and Ibn Umar used to do likewise".
Reference: Sunan an-Nasa'i 743
In-book reference: Book 9, Hadith 2
English translation: Vol. 1, Book 9, Hadith 744