Short answer
Because Allah commands it: zakat is the third pillar of Islam and fard on every Muslim who owns the nisab. The Quran says it purifies the giver and his wealth, and it makes a fixed share of the wealth of the rich a right of the poor, not a favour.
Zakat is fard by the Quran, the Sunnah and the consensus of the scholars. It is an act of worship ('ibadah), so it needs the intention of zakat and must be given for Allah alone, not for thanks, reputation or tax relief.
The four schools agree on this.
Explanation
The first reason is simple: Allah commands it. The Quran joins zakat to prayer again and again, and the Prophet ﷺ named it among the five pillars of Islam. A Muslim gives zakat as worship, in the same spirit as he prays.
The Quran and Sunnah also explain what zakat does:
- It purifies the giver. Allah says zakat purifies the believers and causes them increase (9:103). It trains the heart away from greed and attachment to money.
- It purifies the wealth. A share of your savings belongs to the poor. Once you hand it over, the rest is clean for you.
- It is the right of the poor. The Quran describes the believers as people in whose wealth there is a right for the one who asks and the one who is deprived (51:19). The poor receive zakat with dignity, as their due.
- It brings growth. The Prophet ﷺ said Allah accepts charity from lawful earnings and raises it like a person raises a foal, until it becomes like a mountain.
- It holds the community together. Wealth keeps moving from those who have more than they need to those who have less.
The Sunnah also warns those who hold back. The Prophet ﷺ said that whoever is given wealth by Allah and does not pay its zakat will see it turned into a serpent on the Day of Resurrection. The four schools agree that zakat is fard and that refusing it is a grave sin.
Zakat is not a tax and not a tithe. It is 2.5 percent of wealth you have held for a year, not of income (see do Muslims pay tithes). To see what you owe, use the zakat calculator.
The evidence
Quran, Surah At-Tawba 9:103
خُذْ مِنْ أَمْوَٰلِهِمْ صَدَقَةًۭ تُطَهِّرُهُمْ وَتُزَكِّيهِم بِهَا وَصَلِّ عَلَيْهِمْ ۖ إِنَّ صَلَوٰتَكَ سَكَنٌۭ لَّهُمْ ۗ وَٱللَّهُ سَمِيعٌ عَلِيمٌ ﴿١٠٣﴾
Take, [O, Muhammad], from their wealth a charity by which you purify them and cause them increase, and invoke [Allah 's blessings] upon them. Indeed, your invocations are reassurance for them. And Allah is Hearing and Knowing.
Zakat purifies the giver and brings increase.
Quran, Surah Adh-Dhaariyat 51:19
وَفِىٓ أَمْوَٰلِهِمْ حَقٌّۭ لِّلسَّآئِلِ وَٱلْمَحْرُومِ ﴿١٩﴾
And from their properties was [given] the right of the [needy] petitioner and the deprived.
The poor have a right in the wealth of the believers.
Sahih al-Bukhari 1410
Sahih... من تصدق بعدل تمرة من كسب طيب ولا يقبل الله إلا الطيب ...
"Whoever gives in charity the equal of a date from lawful earnings, and Allah accepts only what is good, Allah accepts it with His right hand and raises it for its giver as one of you raises a foal, until it becomes like a mountain."
Charity from lawful wealth grows with Allah.
Sahih al-Bukhari 1403
Sahih... من آتاه الله مالا فلم يؤد زكاته ...
"Whoever is given wealth by Allah and does not pay its zakat, his wealth will be made into a bald serpent for him on the Day of Resurrection."
A warning for those who withhold zakat.
Where the ruling is recorded
- Bada'i al-Sana'i, Kitab al-Zakat, on the proof of its obligation. Sets out the obligation from the Quran, Sunnah and consensus, and the reasons behind it: thanks for blessings, purification and help for the poor.
- al-Hidaya, Kitab al-Zakat, opening section. Zakat is fard and requires intention.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.