Sahih Bukhari | Volume 9 | Hadith Number 388
Narrated by Abu Huraira
When Allah's Apostle died and Abu Bakr was elected as a Caliph after him, some of the Arabs reverted to disbelief, 'Umar said to Abu Bakr, "How dare you fight the people while 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 says: None has the right to be worshipped but Allah.' waves his wealth and his life from me unless he deserves a legal punishment lusty, and his account will be with Allah! Abu Bakr said, "By Allah, I will fight him who discriminates between Zakat and prayers, for Zakat is the Compulsory right to be taken from the wealth By Allah, if they refuse to give me even a tying rope which they use to give to Allah's Apostle, I would fight them for withholding it." 'Umar said, 'By Allah, It was nothing, except I saw that Allah had opened the chest of Abu Bakr to the fight, and I came to know for certain that was the truth."