Sahih al-Bukhari 498
Sahih al-Bukhari · Book 8: Prayers (Salat) · To offer As-Salat (the prayer) using a Harba (a short spear) (as a Sutra)
Narrated `Abdullah:
حَدَّثَنَا مُسَدَّدٌ ، قَالَ حَدَّثَنَا يَحْيَى ، عَنْ عُبَيْدِ اللَّهِ ، أَخْبَرَنِي نَافِعٌ ، عَنْ عَبْدِ اللَّهِ ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يُرْكَزُ لَهُ الْحَرْبَةُ فَيُصَلِّي إِلَيْهَا.
The Prophet (ﷺ) used to get a Harba planted in front of him (as a Sutra) and pray behind it.