Narrated Abu Huraira (radiallaahu `anhu):
Abu Bakr (radiallaahu `anhu) said, “By Allah! If they (pay me the Zakat and) withhold even a she-kid which they used to pay during the lifetime of Allah’s Apostle (sallallaahu `alayhi wasallam), I will fight with them for it.” ‘Umar (radiallaahu `anhu) said, “It was nothing but Allah Who opened Abu Bakr (radiallaahu `anhu)‘s chest towards the decision to fight, and I came to know that his decision was right.”
As the news of the Prophet’s death spread, a number of tribes rebelled and refused to pay Zakat (poor-due), saying that this was due only to the Prophet . At the same time a number of impostors claimed that the prophethood had passed to them after Muhammad and they raised the standard of revolt. To add to all this, two powerful empires, the Eastern Roman and the Persian, also threatened the new-born Islamic state at Medina.
Under these circumstances, many Companions of the Prophet, including Umar, advised Abu Bakr to make concessions to the Zakat evaders, at least for a time. They told him to accept their prayer from them and leave it up to Allah to judge them for denying Zakat. The new Caliph disagreed. He insisted that the Divine Law cannot be divided, that there is no distinction between the obligations of Zakat and Salat (prayer), and that any compromise with the injunctions of God would eventually erode the foundations of Islam. Umar and others were quick to realize their error of judgment. The revolting tribes attacked Medina but the Muslims were prepared. Abu Bakr himself led the charge, forcing them to retreat. He then made a relentless war on the false claimants to prophethood, most of whom submitted and again professed Islam.
The threat from the Roman Empire had actually arisen earlier, during the Prophet’s lifetime. The Prophet had organized an army under the command of Usama, the son of a freed slave. The army had not gone far when the Prophet had fallen ill so they stopped. After the death of the Prophet , the question was raised whether the army should be sent again or should remain for the defense of Medina. Again Abu Bakr showed a firm determination. He said, “I shall send Usama’s army on its way as ordered by the Prophet, even if I am left alone.”