I would like to know about the justifications for [khums](http://en.wikipedia.org/wiki/Khums) as understood by Shi'a scholars. Their definition doesn't seem to appear in Quran, so I am wondering where it comes from. In particular, do we know if the Prophet or Ali (PBUT) collected khums (in this sense) or said anything about it? *I am only interested in the views of Shi'a scholars and hadith accepted by them. Please cite name of the scholar(s) who have expressed the opinion and cite references when possible.*

What's the difference between the viewpoint of Shia and Sunni about Khums? And why? Although the verse from which the rule of Khums is clear, the idea of Shia and Sunni about this rule is completely different.