أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنِ ابْنِ عُمَرَ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُصَلِّي عَلَى رَاحِلَتِهِ فِي السَّفَرِ حَيْثُمَا تَوَجَّهَتْ بِهِ ‏.‏ قَالَ مَالِكٌ قَالَ عَبْدُ اللَّهِ بْنُ دِينَارٍ وَكَانَ ابْنُ عُمَرَ يَفْعَلُ ذَلِكَ ‏.‏

It was narrated that Ibn 'Umar said: "The Messenger of Allah (PBUH) used to pray on his mount while on a journey, no matter what direction it was facing."Malik said: "Abdullah bin Dinar said: 'And Ibn 'Umar used to do likewise.'" Translation Not Available

Book Ref: Sunan An Nisai Book 5 Hadith 492
Web Ref:  Sunan An Nisai Vol 1 Book 5 Hadith 493