Clooney's Italian wedding to Amal Alamuddin has been in news recently but the couple is reportedly selling their photos to a magazine to raise money for the charity.
Harris, who was happy showing off pictures from his marriage, joked he got married in Italy before Clooney.
"Take that, Clooney. I don't see you showing pictures of your wedding on talk shows," Harris said on David Letterman's Late Show.
Harris said they chose the destination because it was the first trip the couple took together. It was just one of many connections to firsts for the couple, that has been together for more than a decade.