If you are referring to the act of quoting or mentioning a source, the correct spelling is "cites." This is the third person singular present tense of the verb "cite."
If you are referring to a city, the correct spelling is "cities." This is the plural form of the noun "city."
For example:
* "The author cites several sources in his research paper." (verb form)
* "There are many large cities in the United States." (noun form)