MLA In-Text Citation?

I am writing a paper and I would like to know how to cite something. In a speech by Robert Kennedy he quotes a Greek poet named Aeschylus. Now in my paper I quoted the line RFK used, so how do I cite, in-text, being it was Aeschylus' quote? I found a book that has the quote in it, but it was written by someone else. Should I just use the author of that book's last name?