Sahih Muslim Book 001, Hadith Number 0029.
Chapter : Command for fighting against the people so long as they do not profess that there is no god but Allah and Muhammad is His Messenger.
It is narrated on the authority of Abu Huraira that when the Messenger of Allah (may peace be upon him) breathed his last and Abu Bakr was appointed as his successor (Caliph), those amongst the Arabs who wanted to become apostates became apostates. 'Umar b. Khattab said to Abu Bakr: Why would you fight against the people, when the Messenger of Allah declared: I have been directed to fight against people so long as they do not say: There is no god but Allah, and he who professed it was granted full protection of his property and life on my behalf except for a right? His (other) affairs rest with Allah. Upon this Abu Bakr said: By Allah, I would definitely fight against him who severed prayer from Zakat, for it is the obligation upon the rich. By Allah, I would fight against them even to secure the cord (used for hobbling the feet of a camel) which they used to give to the Messenger of Allah (as Zakat) but now they have withheld it. Umar b. Khattab remarked: By Allah, I found nothing but the fact that Allah had opened the heart of Abu Bakr for (perceiving the justification of) fighting (against those who refused to pay Zakat) and I fully recognized that the (stand of Abu Bakr) was right.