Unfortunately, all the news sources appeared to have cited one another and nobody provided a reference.

However, we do know that Fernando Alvarez de Toledo, the 3rd Duke of Alba, did command an Imperial Army on behalf of Philip II of Spain against Pope Paul IV's Papal troops in 1556-7.

Pope Paul IV had confiscated the estates of the Colonna family, who were allied to Philip II, by declaring them rebels. Further, the Pope stripped Philip II's title as King of Naples. While it was a clearly hostile act, both Philip II and the Duke of Alba approached cautiously as the previous time that Imperial forces had to fight against the Pope, their troops mutinied due to lack of pay and sacked Rome in 1527. The result was embarrassment and loss of prestige for Charles V.

After a campaign in which Paul IV allied with France, the Duke of Alba prevailed and formalized a peace treaty in September 1557. Paul IV recognized Philip II as King of Naples, and in return the Duke of Alba presented the pope with a white horse to symbolize the feudal obedience of the King of Naples to the pope.

Year later, Pope Pius V was a Spanish-Imperial ally and supported Philip II's decision to send Alba at the head of a Spanish-Imperial contingent to the Low Countries in response to outbreak of rioting by Calvinist mobs. Several pieces of art were made showing Alba receiving a sword from the Pope, and receiving blessing.

Source: Henry Kamen's "The Duke of Alba," Yale University Press, 2004.