Sahih Bukhari Volume 002, Book 023, Hadith Number 483.
Narated By Abu Huraira : When Allah's Apostle died and Abu Bakr became the caliph some Arabs renegade (reverted to disbelief) (Abu Bakr decided to declare war against them), 'Umar, said to Abu Bakr, "How can you fight with these people although Allah's Apostle said, 'I have been ordered (by Allah) to fight the people till they say: "None has the right to be worshipped but Allah, and whoever said it then he will save his life and property from me except on trespassing the law (rights and conditions for which he will be punished justly), and his accounts will be with Allah.' " Abu Bakr said, "By Allah! I will fight those who differentiate between the prayer and the Zakat as Zakat is the compulsory right to be taken from the property (according to Allah's orders) By Allah! If they refuse to pay me even a she-kid which they used to pay at the time of Allah's Apostle. I would fight with them for withholding it" Then 'Umar said, "By Allah, it was nothing, but Allah opened Abu Bakr's chest towards the decision (to fight) and I came to know that his decision was right."