The information came from Israeli intelligence services, the Mossad.
We'd also note that Hezbollah was among the first to celebrate Obama's one-sided deal (coercion) with Castro on December 17th, 2014.
Here is the text of the email to Secretary Clinton from her long-time advisor, Sidney Blumenthal:
During the week of September 5, 2011 extremely sensitive sources reported in confidence that the Israeli Intelligence and Security Service (Mossad) has informed the leadership of the Israeli Government that Hezbollah is establishing an operational base in Cuba, designed to support terrorist attacks throughout Latin America. While this operation is aimed particularly at Israeli diplomatic and business interests, these sources believe that Hezbollah supporters have been instructed to also begin casing facilities associated with the United States and the United Kingdom, including diplomatic missions, major banks, and businesses in the region. These individuals believe that the Hezbollah military commanders in Lebanon and Syria view these U.S. and U.K. entities as contingency targets to be attacked in the event of U.S. and British military intervention in either Syria or Iran, at some point in the future.
The Hezbollah office in Cuba is being established under direct orders from the current General Secretary Hasan Nasrallah, who replaced Musawi in 1992. According to the information available to this source, in preparation for establishment of the base, Nasrallah, working from inside of Lebanon, carried out secret negotiations with representatives of the Cuban Government, particularly the Cuban Intelligence Service (General Intelligence Directorate — DGI), agreeing to, maintain a very low profile inside of Cuba. Nasrallah also promised to take measures to avoid any trail of evidence that could lead back to Cuba in the event of a Hezbollah attack in Latin America.