What is the ruling concerning those who withhold Zakah?

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The Companions agreed to fight those who refused to pay Zakah. Abu Bakr fought those Arabs who renounced their obligations after the passing of the Prophet (peace be upon him). When challenged regarding the Hadith, "I have been commanded to fight the people until they say La ilaha illa Allah, so whoever says it has protected their wealth and life from me except by its right, and their reckoning is with Allah," Abu Bakr declared he would fight those who made a distinction between prayer and Zakah, asserting that Zakah is the right of the wealth. He stated he would fight them even if they withheld a 'Anāq (a young goat) or an 'Iqāl (a rope used for tethering, interpreted as the minimum due in charity), which they used to render to the Messenger of God.