The source at the Miami Institute indicated that "An Arab military officer at the Damascus airport reportedly witnessed two Russian planes arrive there with Cuban military personnel on board. When the officer questioned the Cubans, they told him they were there to assist Assad because they are experts at operating Russian tanks."
It is unclear what nationality the "Arab" officer was. Perhaps, said Arab determined the people aboard the Russian plane were Cubans because he saw them smoking cigars and drinking mojitos. The Cuban soldiers then volunteered - supposedly - they were "there to assist Assad" because of their expertise manning Russian tanks. However improbable this may seem to an unbiased observer, the source from the Miami Institute said that "it doesn't surprise me."
The supposed U.S. official - who Fox grants anonymity to without giving a reason why - related "evidence" from "intelligence reports" that Cuban troops "may" have trained in Russia and "may have" come to Syria in Russian planes. Sounds legit.
Despite the thinness of the report's sourcing and the improbability of its content, other news organizations were quick to parrot its claims. Spanish newspaper ABC noted the next day that media from Germany to Argentina to the Middle East had echoed the Fox News report, while ABC did the same themselves.
By Friday, the story had gained enough traction that it was raised at a White House briefing. In a response that should have been enough to put the story to rest, the White House Press Secretary said "we've seen no evidence to indicate that those reports are true."
But a few hours later, the Daily Beast had definitively declared in a headline that: "Cuba Is Intervening in Syria to Help Russia. It's Not the First Time Havana's Assisted Moscow."
Progressive concern troll James Bloodworth turned Fox's rumors into fact and wrote that "Not for the first time Cuban forces are doing Russia's dirty work, this time in Syria... Obama has been holding his hand out in a gesture of goodwill to America's adversaries only for them to blow him a raspberry back in his face - while standing atop a pile of Syrian corpses."
In reality, Obama's "gesture of goodwill" is little more than behaving less overtly hostile after decades of American aggression against Cuba and Iran. If you are choking someone unprovoked and you loosen you're grip, it is far from a gesture of goodwill.
Bloodworth also tries to make a historical argument that Cuba's (imaginary) military actions in Syria are consistent with their "bloody" interventions elsewhere. He decries "Cuban terror in Ethiopia" that resulted in hundreds of thousands of people being killed. "The tragedy was largely a consequence of the policies pursued by the Communist dictatorship that ruled Ethiopia at the time - a regime propped up by Cuba and the Soviet Union."
In 1977, Somalia had invaded Ethiopia in an attack that "had been encouraged by ambivalent signals from Washington," according to historian Piero Gleijeses in his book Visions of Freedom.  Initially reluctant to become involved, Fidel Castro finally agreed to Ethiopian requests to send troops to repel the Somali invasion.
Gleijeses found in his extensive review of formerly classified military documents that Cuba's motives in aiding Ethiopia were sincere:
With hindsight, we know that Mengistu's policies resulted in disaster, but this was not clear in 1977: though the process was undeniably bloody, the Ethiopian junta had decreed a radical agrarian reform and taken unprecedented steps to foster the cultural rights of the non-Amhara population... The evidence indicates that the Cubans intervened because they believed, as Cuban intelligence stated in March 1977, that 'the social and economic measures adopted by Ethiopia's leadership are the most progressive we have seen in any underdeveloped country since the triumph of the Cuban revolution.' In addition to correcting the record on Ethiopia, Gleijeses' study also serves to set the record straight on Cuba's historical modus operandi in its military interventions abroad. Cuba did maintain a large military presence in Angola for nearly 15 years, starting in 1975.
Castro first sent troops in November 1975 after Angolan President Agostinho Neto warned of a South African invasion of the country already underway which would inevitably topple the nascent government without outside support. Cuba agreed to send soldiers to Angola right away. Several months later, they would repel the apartheid army back to Pretoria. They remained in Angola at Neto's bequest to prevent further incursions from the racist South African army into the country's sovereign territory.
At the same time, there was an ongoing civil war between Neto's MPLA, the largest and most popular of the guerilla groups, and the South African and American-backed UNITA guerillas led by former Portuguese collaborator Jonas Savimbi.
Castro was adamant that Cuban troops would be responsible for preventing a South African invasion, while Angolan troops should deal with their own internal conflict. In meetings with Neto, Castro "kept hammering away on the need to fight the bandits ... He explained to us that the fight against the bandits was necessarily and without question the responsibility of the Angolans, that we could not wage this war, that it was their war." 
Cuba's position during the Angolan conflict is consistent with the diplomatic approach they have repeatedly espoused in Syria, that the Syrian conflict is a domestic problem for the Syrian people and government to resolve themselves, while the international community works to achieve a peaceful solution.
"Cuba reiterates that international cooperation, based on the principles of objectivity, impartiality and non-selectivity, is the only way to effectively promote and protect all human rights," Cuban representative to the UN Human Rights Council Rodolfo Reyes said at a meeting in Switzerland. He added that "Cuba is confident of the capacity of the Syrian people and government to solve their domestic problems without foreign interference."
That the Fox News could cause such a stir is a testament to the refusal of mainstream news organizations to verify sources. In all of the iterations of the "Cuban troops in Syria" fantasy, there are no new sources cited. The original Fox News report cites one anonymous U.S. official who may, or may not, even exist. The only source on record with their incredulous claims is someone from the Institute for Cuban and Cuban-American Studies (ICCAS) at the University of Miami.
ICCAS is notorious for its reactionary, anti-Communist politics revered among the fanatically right-wing Cuban and Cuban-American population in Miami. Their academic research includes a conspiracy theory that appears to implicate Fidel Castro in the assassination of John F. Kennedy.
Another ICCAS report claims "the often-repeated view in many countries that the United States is an evil power, guilty for much of the problems and sufferings of the developing world, is owed in great part to the propaganda efforts of Fidel Castro" - not, rather, to decades of direct U.S. military intervention; profligate support to fascist military dictatorships; and predatory, neo-colonial lending policies that demand neoliberal structural adjustment programs which funnel public assets and resources to creditor interests, at the expense of the employment, health and well-being of the vast majority of local populations.
ICCAS is also home to the Cuba Transition Project whose mission is "to study and make recommendations for the reconstruction of Cuba once the post-Castro transition begins in earnest." CTP acknowledges on its Web site that "the project was established in 2002 and supported by grants from the U.S. Agency for International Development (USAID) until 2010." It's funding indicates it is at least indirectly an arm of the U.S. government's destabilization and subversion efforts dedicated to regime change of the politically and economically independent Cuban government.
Cuban Prensa Latina reporter in Syria Miguel Fernández noted that ICCAS has reported six or seven times since 2006 that Fidel Castro has died. He suggested reports such as those originating with ICCAS about Cuban troops in Syria were part of the campaigns of reactionary groups opposed to normalization to tarnish the new relations between Cuba and the United States.
The Cuban Embassy in Damascus reportedly "laughed" at the report of Cuban troops in Syria, and told Sputnik News: "It's pure lunacy. It is as if they were claiming that Russia had sent its troops to Madagascar to protect lemurs."
Despite claims of Cuban troops in Syria contradicting Cuba's stated policy and historical modus operandi, and the fact that now four days have passed without a single piece of corroborating evidence to the laughable Fox News report, the imaginary Cuban troops in Syria are likely to morph into more outrageous fantasies of media who have shown themselves primarily interested in fabricating tales of intrigue about America's evil enemies rather than reporting actual verifiable facts.
 Gleijeses, Piero. Visions of Freedom: Havana, Washington, Pretoria, and the Struggle for Southern Africa, 1976-1991. The University of North Carolina Press, 2013. Kindle edition.
 as quoted in Gleijeses, 2013