Assad has many times. Nasser did, off and on (depending on which way the wind was blowing - friendly with the Americans, economy doing well, he'd ignore the issue. At odds with the Americans, war in Yemen going badly, economy in the tank, call for the destruction of Israel). Assad and various of his underlings in Syria have frequently. I believe some statements came out of Lebanon before it blew itself all to sh-- in the '80s, and I'm bloody sure Hussein's Iraq has made such statements as well.
Mostly, though, such statements are to mollify the masses. Much like the aforementioned annual Chinese saber-rattling towards Taiwan has historically been: all bark, no bite.