I am writing a short article on the $\zeta$-function, and I use the "Rising factorial" representation of the function in the article. You can see that in that here. Now there is no citation in wikipedia concerning that identity. So, if I use that identity in my article, how should I cite that?
Ideally, Wikipedia would link to a source for the result, the source would be accessible to you, and you could cite it instead. But it often happens that Wikipedia doesn't have a source. In that case, be warned that Wikipedia may be wrong. In any case, many people do not consider Wikipedia an acceptable source. But if you want to cite Wikipedia anyway (you're sure the result is right, or it's an informal article, or you're specifically pointing out how something is treated on Wikipedia, etc.), Wikipedia has tools showing you how to cite it. In the left sidebar of Wikipedia, under "Toolbox", the last link is called "Cite this page".
Click on it. You'll be taken to a page like this which tells you how to cite it in various formats (APA style, MLA style, etc.), including (most useful if you're using LaTeX) the BibTeX entry. It also gives you a permanent link to the version as you saw it, so that even if the section is later changed on Wikipedia, those who follow your citation will see exactly what you saw (assuming of course that the article itself doesn't get deleted).
If I were you, I would search the literature for a citation from a different source. Wikipedia is usually not considered "good" source for citations.
If you insist, I would have cited it as:
"Riemann zeta function", Wikipedia, the free encyclopedia, http://en.wikipedia.org/wiki/Riemann_zeta_function#Representations