If you took the statistic from a website, you cite it as you would any other website. The same goes for statistics found in books, journals, magazines or databases—just follow the usual citation method for each source.
The Owl at Purdue also has a great article on Writing with Statistics, outlining the best ways to use other's data and how to interpret it.
Check out Flowing Data Blog, which shows " how statisticians, designers, data scientists, and others use analysis, visualization, and exploration to understand data and ourselves(website)."