Wikipedia is not a mirror of public domain or other source material. In Wikipedia articles, quotes of any original texts being discussed should be relevant to the discussion (or illustrative of style) and should be kept to an appropriate length.
Fair use allows us to quote short sections from copyrighted primary sources, if relevant to explaining the topic of an article. For instance, we can quote a sentence or two from a movie review in an article on the movie, or quote a small portion of a modern poem in order to illustrate its style. In all such cases, be sure to properly attribute the work to its original author, and state where it was published. When writing an article at Wikipedia about a copyrighted primary source, please follow Wikipedia's Non-free content policy carefully before including quotes from the copyrighted source.
If moving text from Wikipedia to Wikisource, make sure that the work is free of copyright, or appropriately licensed.
For example, older lyrics of the Internationale have entered the public domain, but the modern version by Billy Bragg is still copyrighted, so quotes from it would need to be short, relevant, properly credited to Billy Bragg, and could not be used on Wikisource except as part of an appropriately licensed primary text on Billy Bragg. Per Wikisource's inclusion policy, the text discussing Billy Bragg would also have needed to pass through editorial oversight or peer review.