Sahih Bukhari
Virtues of the Prayer Hall (Sutra of the Musalla)
Volume: 001 / Book Number: 009 / Hadith Number: 473
Narrated Ibn 'Umar:
Whenever Allah's Apostle came out on 'Id day, he used to order that a Harba (a short spear) to be planted in front of him (as a Sutra for his prayer) and then he used to pray facing it with the people behind him and used to do the same while on a journey. After the Prophet , this practice was adopted by the Muslim rulers (who followed his traditions).