Sahih Bukhari Volume 009, Book 084, Hadith Number 059.
Narrated By Abu Huraira : When the Prophet died and Abu Bakr became his successor and some of the Arabs reverted to disbelief, 'Umar said, "O Abu Bakr! How can you fight these people although Allah's Apostle said, 'I have been ordered to fight the people till they say: 'None has the right to be worshipped but Allah, 'and whoever said, 'None has the right to be worshipped but Allah', Allah will save his property and his life from me, unless (he does something for which he receives legal punishment) justly, and his account will be with Allah?' "Abu Bakr said, "By Allah! I will fight whoever differentiates between prayers and Zakat as Zakat is the right to be taken from property (according to Allah's Orders). By Allah! If they refused to pay me even a kid they used to pay to Allah's Apostle, I would fight with them for withholding it." 'Umar said, "By Allah: It was nothing, but I noticed that Allah opened Abu Bakr's chest towards the decision to fight, therefore I realized that his decision was right."