Fiqh-us-Sunnah - 3.1

Hadith 3.1.1

Zakah or alms tax can be defined as that portion of a man's wealth which is designated for the poor. The term is derived from the Arabic verbal root meaning "to increase." "to purify," and "to bless." It find its origin in Allah's command to: "Take sadaqah (charity) from their property in order to purify and sanctify them" [at-Taubah 103]. That is why this kind of sadaqah is called zakah, for by paying it, one is aspiring to attain blessing, purification, and the cultivation of good deeds.
Taking into account its very nature, it is no wonder that zakah constitutes one of the five pillars of Islam. It is associated with prayer (salah) in eighty-two Qur'anic verses. Allah, the Exalted One, prescribed it in His Book (The Qur'an), His Messenger corroborated it by his (sunnah), and the community (ummah) by consensus upheld it. Ibn 'Abbas reported that when the Prophet, upon whom be peace, sent Mu'adh ibn Jabal to Yemen (as its governor), he said to him: "You are going to a people who are People of the Scripture. Invite them to accept the shahadah: that there is no god but Allah and I am His Messenger. If they accept and affirm this, tell them that Allah, the Glorious One, has enjoined five prayers upon them during the day and night. If they accept that, tell them also that He has enjoined sadaqah upon their assets which will be taken from the rich of the (Muslim) community and distributed to the poor. If they accept that, refrain from laying hands upon the best of their goods and fear the cry of the oppressed, for there is no barrier between Allah and it."
At-Tabarani relates in al-'Awsat and as-Saghir, on the authority of 'Ali, that the Prophet said: "Allah has enjoined upon rich Muslims a due to be taken from their properties corresponding to the needs of the poor among them. The poor will never suffer from starvation or lack of clothes unless the rich neglect their due. If they do, Allah will surely hold them accountable and punish them severely." According to at-Tabarani: "It was reported only by Thabit ibn Muhammad az-Zahid." Of Thabit's credibility, al-Hafiz in turn says: "Thabit was an honest and trustworthy person. AlBukhari and others related from him, and the rest of the narrators in the chain are considered as accepted authorities."
In the early days of Islam at Makkah, no limit or restriction was placed on the amount to be donated, for that decision was left to the individual Muslim's conscience and generosity. In the second year of hijrah, according to the widely known authorities, both the type and the quantity of zakah revenues were determined, and detailed illustrations were provided.