What is the Shia justification for khums?

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.*