Sunan an-Nasa'i, Volume 1, Book 9, Hadith 748 (DarusSalam) ⓘ
Narrated Ibn Umar: It was narrated from Ibn Umar concerning the Messenger of Allah he said: "He used to set up a short spear then pray facing toward it."
أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ، قَالَ أَنْبَأَنَا نَافِعٌ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ كَانَ يَرْكُزُ الْحَرْبَةَ ثُمَّ يُصَلِّي إِلَيْهَا .
also referenced under: Hadith 747 (Reference) & Book 9, Hadith 6 (In-Book)