One assumes that you are not quoting a really lengthy section (pages and pages), so the doctrine of fair use would apply, because you are quoting so that you and others can comment on it which is one of the acceptable categories of fair use. Use quotation marks and reference the source as tinyfaery said.
Here is an excerpt from the copyright office just so you have it for reference:
“Section 107 contains a list of the various purposes for which the reproduction of a particular work may be considered “fair,” such as criticism, comment, news reporting, teaching, scholarship, and research. Section 107 also sets out four factors to be considered in determining whether or not a particular use is fair:
1.
the purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes;
2.
the nature of the copyrighted work;
3.
amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
4. the effect of the use upon the potential market for or value of the copyrighted work.”