Sahih Muslim Book 4, Hadith Number 1011.

Chapter : The excellence of the prayer and the way it is begun and the excellence of Ruku’ and moderation in it and prostration and moderation in it, etc.

Ibn Umar reported: The Apostle of Allah (may peace be upon him) set up (sutra), and Abu Bakr said: He implanted iron-tipped spear and said prayer towards its direction. Ibn Abu Shaiba made this addition to it: “Ubaidullah said that it was a spear.”