Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1011
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."