The issue of Khums has only been mentioned once in holy Qur'an (and also it is not only Khums which has been mentioned in one verse, but there are lots of rulings of Qur'an which have been mentioned only once in Qur'an and there is no doubt that, this one time expression is enough). And that is verse 41 of Anfal Sura:

وَ اعْلَمُوا اَنَّما غَنِمْتُمْ مِنْ شَىْءه خُمُسَهُ وَ لِلرَّسُولِ وَ لِذِى الْقُرْبى وَ الیَتامى وَ الْمَساکینِ وَ ابْنِ السَّبیلِ اِنْ کُنْتُمْ آمَنْتُمْ بِالله وَما اَنْزَلْنا عَلى عَبْدِنا یَوْمَ الْفُرْقانِ یَوْمَ الْتَقَىالْجَمْعانِ وَ الله عَلى کُلِّ شَىء قَدیرٌ

And know that whatever advantage you gain from things, one fifth of it is for Allah and Prophet (s.a.) and his Ahl -e- Bait and orphans and needy persons and suspended travelers (from them), if you have believed in Allah and the thing that he descended to his servant on the day of distinction of truth from evil, the day of confronting of two groups (believer and unbeliever; the day of battle of Badr); and Allah is powerful over all things.

Important matter which should be noticed in this verse and in fact all the issue concentrates on it is that does the advantage which has been mentioned in this verse only consist of spoils of war or expands to any kind of income.

In the first form, this verse only expresses the spoils of war and for Khums of other things we should use tradition, news and correct and reliable cabbalas and there is no problem in that Qur'an points to a part of ruling in proportion and other parts express in tradition.

For example, quintet daily prayer has been mentioned explicitly in holy Qur’an, and also it has been pointed to the prayer of Tawaf (circumambulate the Ka’ba) which is one of obligatory prayers, but nothing has been mentioned about Ayaat (signs) prayer which is agreed by all Muslim, Shiites and Sonnies, and Ghazaa prayer and like them; and we cannot find anyone who says because Ayaat prayer has not been mentioned in Qur'an and it is only available in tradition of Prophet (s.a.) should not be performed, or because it has been pointed to some Ghusls (full ablution) in Qur'an and nothing has been said about some other Ghusls, therefore we should abstain from them. This is a logic which no Muslim accepts.

Therefore, there is no problem that Qur'an has expressed only a part of cases of Khums and has leaved the others to tradition, and there are lots of issues like this in Islamic Fiqh and those who know even a little about issue of Fiqh have understood this matter completely; but we should find that what the meaning of advantage (غنیمت) literally and in commonsense is.

Indeed, is it exclusive to spoils of war or it consists of any kind of income?

The definition which derives from dictionaries is that in the literal root of this word there is nothing about war or the things which is gained from enemy, but it consists of any kind of income. We refer to some parts of famous dictionaries as evidence which are all accepted by Arab scientist and litterateurs:

We read in the book “Lisan Al-Arab” (Tongue of Arab), vol. 12:

و الغنم: الفوز بالشّىء من غیر مشقّة و... الغنم،الغنیمة و المغنم، الفىء... و فى الحدیث الرّهن لمن رهنه له غنمه و علیه غرمه، غنمه زیادته و نمائه و فاضل قیمته... و غنم الشّىء غنما: فاز به

“غنم” means gaining a thing without any hardship and “غنم”, “غنیمت” (advantage) and “مغنم” are all in the meaning of “فیء” (and also “فیء” has been meant literally as the things which reach a person without labor …) and it has been said in Hadith that pawn is for the person who has taken that pawn and its advantage (غنیمت) and benefit is for him, and also its compensation and loss is for him, and “غنم” means the excess, growth and surplus of the price … he take that as an advantage (غنیمت) means that he gained that …

And we read in the book “Taj Al-Arous” (the bride’s brown), vol. 9:

والغنم الفوز بالشیء بلا مشقّة!

Advantage (غنیمت) is the thing that person gains without hardship!

And also in the book “Al-Qamoos” it has been mentioned with the same meaning.

And in the book “Mufradat” by Ragheb “غنیمت” has been said to be derived from the root “غنم” in the meaning of “sheep” and then it says:

ثم استعمل فى کل مظفور به من جهة العدى و غیرهم

Then it has been used in all things which person gains from enemy or non-enemy.

Even persons who consider one of the definitions of “غنیمت” as spoils of war do not deny that its main meaning is more expanded which implies to all things that a person can gain without hardship.

In common use advantage “غنیمت” is used against Fine “غرامت” and as the meaning of fine is an expanded meaning and consists of any kind of fine, also advantage “غنیمت” has an expanded meaning and implies to any kind of notable income.

This word has been used in many cases in “Nahjul Balagha”:

We read in sermon 76:

اِغْتَنَمِ الْمَهَلْ

Take advantage from opportunities.

And we read in sermon 120:

مَنْ اَخَذَ بِها لَحِقَ وَ غَنِمَ

Person who acts as the religion of Allah finds happiness and takes advantages.

And he says in letter 53 to Malik Ashtar:

وَ لا تَکُونَنَّ عَلَیْهِمْ سَبُعاً ضارِیاً تَغْتَنِمُ اَکْلَهُمْ

Do not be like a cruel animal against people of Egypt in the way that consider eating them as an advantage غنیمت” and a kind of income for yourself!

And he says in letter 45 to Osman ibn Hanif:

فَوَ الله ما کَنَزْتُ مِنْ دُنْیاکُم تِبْرأَ وَلا اَدَّخَرْتُ مِنْ غَنائِمِها وَفْراً

I swear to Allah that I did reserve no gold from your world, and I did no saving from its advantages or incomes!

And it is in quotes, in sentence 331:

اِنَّ الله سُبْحانَهُ جَعَلَ الطّاعَةَ غَنِیمَةَ الاْکْیاسِ

Indeed pure Allah has made obedience as advantage and benefit of tactful people.

And we read in letter 31:

وَ اغْتَنِمْ مَنِ اسْتَقْرَضَکَ فى حَالِ غِناکَ

If a person asks you for a loan when you are rich, consider this as an advantage غنیمت

And there are lots of expressions like these.