Musnad Ahmad 27:1826

Ibn Numayr narrated to us, he said: Ubaydullah narrated to us, from Nafi’, from Ibn Umar, that the Messenger of Allah (peace and blessings of Allah be upon him) used to, when he went out on the day of Eid, command that a spear be brought, and it would be placed in front of him, and he would pray facing it, while the people were behind him. And he used to do that while traveling. So, from that, the commanders adopted it.