Sunan an-Nasa'i 492

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 (ﷺ) 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

Reference

Sunan an-Nasa'i 492

In-book

Book 5, Hadith 45