2 of 8 formatting and removed the word sunni because question about shia view only

The most important verse in Qur'an which is the basis of decisions about Khums is this verse : 8:41

وَاعْلَمُوا أَنَّمَا غَنِمْتُم مِّن شَيْءٍ فَأَنَّ لِلَّـهِ خُمُسَهُ وَلِلرَّ‌سُولِ وَلِذِي الْقُرْ‌بَىٰ وَالْيَتَامَىٰ وَالْمَسَاكِينِ وَابْنِ السَّبِيلِ إِن كُنتُمْ آمَنتُم بِاللَّـهِ وَمَا أَنزَلْنَا عَلَىٰ عَبْدِنَا يَوْمَ الْفُرْ‌قَانِ يَوْمَ الْتَقَى الْجَمْعَانِ ۗ وَاللَّـهُ عَلَىٰ كُلِّ شَيْءٍ قَدِيرٌ‌

And know that whatsoever ye obtain of spoils then verily unto Allah belongeth a fifth thereof and unto the apostle and unto his kindreds and the orphans and the needy and the wayfarer if ye indeed have believed in Allah and that which We sent down on our bondmans on the day of distinction, the day whereon the two hosts met. And Allah is over everything Potent

The word غنمتم means what you obtain. But Shia say it is about spoil. They say Spoil is a name of any money that the person will make from 7 things:

  1. War.
  2. Diving(finding something from under water).
  3. Treasure.
  4. Mine.
  5. Trade.
  6. Wealth mixed with Haram.
  7. Buying a land from a Kafir.

Although this verse is exactly about the Booty, But the Arab literature scholars say the word غنیمت is a name of all the money that people gather, unless the money that the people gifted.

[Reference 1]

And also there is a Hadith from Imam Javad(A.S.) which shows that the Ahlul bayt belive that the KHums belongs to every money that the human makes from those seven ways.

الامام جواد علیه السلام: : « فاما الغنائم و الفوائد فهى واجبه عليهم فى كل عام قال الله تعالى : و اعلموا انما غنمتم من شىء فان لله خمسه . . . »

Which brings الفوائد close to الغنائم which shows that he believes the money making is equal to booty.

There are some other Hadiths which shows that just believers pay Khums and the Khums can clean the money from Haram. Look ate this Hadith:

الامام رضا علیه اسلام:

« اِنَّ اِخْراجَهُ ( خمس ) مِفْتاحُ رِزْقِكُم و تَمْحيصُ ذُنُوبِكُم »

Paying Khums is a key to expand the wealth and forgiveness of sins.

[Reference 2]