Mukhtasar Bukhari

It is narrated from Sayyiduna Abdullah bin Umar, may Allah be pleased with them both, that when the Prophet Muhammad (peace and blessings be upon him) would go out (to lead the prayer) on the day of Eid, he would order that a harbah (small spear) be fixed in front of him, and he would pray facing towards it, and the people would be behind him. And he (peace and blessings be upon him) would do the same while traveling. From this, the leaders have adopted this practice (i.e., keeping a spear with them).