The more academic your sources are, the more likely you are to encounter this situation!
1. Ideally, you'd locate the source mentioned to use directly.
This is half the reason we have citations -- so that other people can see your sources for themselves, especially if they're following a similar line of research. Think of it as a very slow-moving conversation that lots of people keep joining.
But if that's not feasible for whatever reason...
2. Cite only the article you actually looked at, but note the other source in your writing.
This is what's formally called an indirect or secondary citation. The exact approach will look a little different based on your citation style.
You will need to find the other source details from the first article's citations list at the end in order to do this successfully. You may even have to do a quick search on the other author(s) to contextualize them
Examples for each style follow:
MLA: qtd. in ___
In-text citation:
Urban legends are modern, oral folktales that, according to Jan Harold Brunvand's book The Vanishing Hitchhiker "often depict a clash between modern conditions and some aspect of a traditional life-style" (qtd. in Best and Horiuchi 492).
Works Cited:
Best, Joel, and Gerald T. Horiuchi. “The Razor Blade in the Apple: The Social Construction of Urban Legends.” Social Problems, vol. 32, no. 5, 1985, pp. 488–99. JSTOR, https://doi.org/10.2307/800777.
APA: as cited in ____
In-text citation:
Urban legends are modern, oral folktales that, according to Jan Harold Brunvand's book The Vanishing Hitchhiker "often depict a clash between modern conditions and some aspect of a traditional life-style" (as cited in Best and Horiuchi, 1985, p. 492).
References:
Best, J., & Horiuchi, G. T. (1985). The razor blade in the apple: The social construction of urban legends. Social Problems, 32(5), 488–499. https://doi.org/10.2307/800777
Chicago Style, Notes-Bibliography: Document in the Footnote and Bibliography
In-text and footnote:
Urban legends are modern, oral folktales that "often depict a clash between modern conditions and some aspect of a traditional life-style."1
1. Jan Harold Brunvald, The Vanishing Hitchhiker (New York: Norton, 1981), quoted in Joel Best and Gerald T. Horiuchi, “The Razor Blade in the Apple: The Social Construction of Urban Legends,” Social Problems 32, no. 5 (1985): 492.
Bibliography:
Brunvald, Jan Harold. The Vanishing Hitchhiker. New York: Norton, 1981. Quoted in Joel Best and Gerald T. Horiuchi, “The Razor Blade in the Apple: The Social Construction of Urban Legends,” Social Problems 32, no. 5 (1985): 488-499.