As an obligation upon Muslims, zakah is one of the essential requirements of Islam. If somebody disputed its obligation, he would be outside of Islam, and could legally be killed for his unbelief unless he was a new Muslim and could be excused for his ignorance.
As for the one who refrains from paying it without denying its obligation, he would be guilty of committing a sin. Yet, this act does not place him outside of Islam. It is the ruler’s duty to take zakah from the defaulter by force and rebuke him, provided he does not collect more than the stipulated amount. However, in the views of Ahmad and ash-Shaf’i (in his earlier opinion) the ruler could take half of the defaulter’s money, in addition to the calculated amount of zakah, as a punishment. This view is based on what Ahmad, anNasa’i, Abu Dawud, al-Hakim, and al-Baihaqi have recorded from Bahz ibn Hakim all the way back to his grandfather who said: “I heard the Messenger of Allah, upon whom be peace, say: ‘Whether the camels of the zakah payer are grown or baby camels, it makes no difference in his reward if he gave them willingly. (However,) if someone refrains from paying it, it will be taken from him along with half his property, for it is a right of our Lord, the Blessed and the Exalted, not a right of the house of Muhammad.'”
Asked about its chain, Ahmad ruled it good (hassan). Of Bahz, al-Hakim says: “His traditions are authentic.” Ash-Shaf’i, as alBaihaqi says, did not include it for fiqhi consideration because… “this hadith is not confirmed by the scholars of hadith.”
If some people refrain from paying zakah knowing that it is due and that they can afford to pay, they should be fought until they yield and pay. Al-Bukhari and Muslim report that Ibn ‘Umar heard the Prophet say: “I have been ordered to fight people until they say that none has the right to be worshipped but Allah, and that Muhammad is His Messenger, and they uphold the prayers, and pay the zakah. If they do this, their lives and properties will be safe, except for what is due to Islam, and their accounts are with Allah.”
Abu Hurairah is reported to have said: “When Allah’s Messenger, upon whom be peace, died and Abu Bakr succeeded him as caliph, some Arabs apostatised, causing Abu Bakr to declare war upon them. ‘Umar said to him: ‘Why must you fight these men?’, especially when there is a ruling of the Prophet, upon whom be peace: ‘I have been called to fight men until they say that none has the right to be worshipped but Allah, and whoever said it has saved his life and property from me except when a right is due in them, and his account will be with Allah.’ Abu Bakr replied: ‘By Allah! I will fight those who differentiate between salah and zakah because zakah is the due on property. By Allah! If they withheld even a young she-goat (‘anaq) that they used to pay at the time of Allah’s Messenger, upon whom be peace, I would fight them.’ Then ‘Umar said: ‘By Allah! It was He who gave Abu Bakr the true knowledge to fight, and later I came to know that he was right.’ “
The same hadith narrated by Muslim, Abu Dawud, and atTirmizhi has the following variant: “If they withheld the ‘iqal, the rope of the camel,” instead of “‘anaq, young she-goat.”