Where in the Great Gatsby does it say, "rich girls don't marry poor boys"?

This quote does not appear in the novel, but it is said by Daisy in the 1974 film adaptation of the story. In the film, Daisy visits Jay Gatsby's home for the first time and has a conversation with Jay about why she decided to marry Tom Buchanan. When Jay asks Daisy why she did not wait until he returned from the war to marry him, she responds by saying, "Because rich girls don't marry poor boys Jay Gatsby." While this quote does not appear in the novel, it emphasizes why Jay Gatsby entered the bootlegging business to attain immense wealth. Gatsby is well aware of the fact that Daisy comes from money, and he has no chance of marrying her because of his lower social status. In order to win Daisy's heart, Gatsby hopes to achieve the American Dream in the hopes that his financial prosperity will be enough to persuade Daisy to leave Tom. Unfortunately, Tom exposes Gatsby as a bootlegger, which ruins Jay's chances of being with Daisy.

The quote “rich girls don’t marry poor boys” does not appear in the novel version of The Great Gatsby. This quote appears in at least one of the film versions and comes from a real-life event in F. Scott Fitzgerald’s life.
While this quote doesn’t appear in the novel, it highlights one of the major themes in the story: the carelessness of the rich. This theme is evident in the ending of the novel through the deaths of Jay, Myrtle, and George, which come at the hands of Daisy and Tom (albeit indirectly). It's also evident in how the younger Daisy allowed the poor Jay Gatsby to court her—Daisy’s decision to agree to marry Jay, but to only break his heart for a more suitable husband in Tom Buchanan. 
The line “rich girls don’t marry poor boys,” if uttered in the novel, would take some of the blame away from Daisy and serve as a warning to Gatsby. But she never mentions this, and, up to the day he dies, Gatsby fully believes that he would have Daisy all to himself one day.