Plagiarism: How to cite the citation of your source? [closed]
If you find material in document A that quotes document B, you must reference both documents in your cites.
The rationale for this is that selecting what's important and relevant is work and you must give credit for that work.
From the Yale College Writing Centre:
If the source you’re reading quotes another text, and you want to use that quoted material in your own essay, you must give credit to the author who originally selected the quotation.