Sunan Ibn Majah, Book 5, Hadith 994 (In-Book) ⓘ
It was narrated that Ibn ‘Umar said: “A small spear (Harbah) would be brought out to the Prophet when he was travelling; he would plant it (in the ground) to perform prayer while facing it.”
حَدَّثَنَا مُحَمَّدُ بْنُ الصَّبَّاحِ، أَنْبَأَنَا عَبْدُ اللَّهِ بْنُ رَجَاءٍ الْمَكِّيُّ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ كَانَ النَّبِيُّ ـ صلى الله عليه وسلم ـ تُخْرَجُ لَهُ حَرْبَةٌ فِي السَّفَرِ فَيَنْصِبُهَا فَيُصَلِّي إِلَيْهَا .
also referenced under: Hadith 941 (Reference) & Volume 2, Book 5, Hadith 941 (DarusSalam)