Narrated Abu Huraira:
Abu Bakr said, "By Allah! If they (pay me the Zakat and) with-hold even a she-kid which they used to pay during the life-time of Allah's Apostle, I will fight with them for it." 'Umar said, "It was nothing but Allah Who opened Abu Bakr's chest towards the decision to fight, and I came to know that his decision was right."