Hello,
Found the quote in both of these sources via Google Books:

Mencia, M. (1982). Time was on our side. La Habana: Editora Politica.
Quote is said to be found on page 15 of this book.

Mencia, M. (1993). The fertile prison: Fidel Castro in Batista's jails. Melbourne, Australia: Ocean.
Quote is said to be found on page 36 of this book.

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Also, this quote is mentioned in the following articles:

Castro's Library Pass (Part III)
By Walter Skold
FrontPageMagazine.com | October 14, 2005

http://www.frontpagemag.com/Articles/Read.aspx?GUID=6C7B72D0-D287-4F28-A0F0-46A35B1457F5

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Nat Hentoff/Free Inquiry, Aug./Sept. 2004
Nat Hentoff: Castro's Gulag and American librarians

In Op-Ed

http://www.secularhumanism.org/index.php?section=library&page=index_24