"Gods says the Holy Prophet, "Take from the properties of your followers a 'sadaqa' i.e., a portion in charity will purify them and keep their hearts clean." (Holy Quran, Surah 9, Ayat 103). It is also called 'zakat' because in the words of the Holy Quran, it meant for the 'tazkiah', i.e., the purification of the hearts of the Muslims.
God has made it obligatory on all Muslims to take out the 'zakat' from their properties and hand it over to the Prophet or his descendants. In the life time of the Prophet, the Muslims used to pay 'zakat' to the officers who were appointed by him for this purpose. They were the persons in charge of the collections who have been mentioned by God in His book.
O momins, if any one of you is put to test by the Imam, then let him bear the trial with patience. The easiest of such trials is the test of money. You must bear in mind that no time is fixed for the Imams for such trials. They can test their followers at any time and in any way they like when they want to raise them to a high position. They are empowered to do what they like. There is no limit to their power. Your duty is to submit to them voluntarily. God says (in the Holy Quran), "he who volunteers to do good things, does best things for himself." (Surah 2, Ayat 184).
Imam Jafar Sadiq, greetings be on him, says "None of the duties of God on man is more severe than the return of 'amanat' to Him from your property. Many people are ruined by the neglect of this duty. See that you put back this 'amanat' in the hands of the Imam of your time. It is not a part of your property." With all this, if one does not pay the 'amanat' and leaves it behind for his heirs, then he is the greatest loser. He loses everything. Not only does he lose his prestige but he also loses his faith. God says ( in the Holy Quran), "When death approached to one of them, he said 'O God, give me back the life in order that I may do well what I have neglected to do." It is nothing but a meaningless expression." (Surah 23, Aya 99-100). Imam Jafar Sadiq, peace be upon him, says that "the words 'what I neglected to do' in the above verse of the Quran refer to the non-payment of the dues of the Imam of the time which God has enjoined to pay. If one does not pay the 'zakat' his prayers will not be recognized.
In conclusion, O momins, subdue yourselves. Do not allow yourselves to come in the way of the discharge of your duties. You must realize that you have to pay only a small portion of what God has given you and that too is God's property which He has entrusted with you. There is no way out of it."