The reference is found in the first chapter of the book, also citing Plato's complaint about Hellenic youth found in the seventh book of Laws, by Plato. However. I could not find a source on it. Is it a false claim? Was Sagan relying on a false claim to do that affirmation?
You may wish to look at a previous answer by /u/methinks2015 on this subject, from 2012.
In short, there does not seem to be a reliable source for the quote.