Sources tell us Mario was working out at the Planet Hollywood Spa by Mandara in Vegas Friday and as he was leaving the bathroom in the spa, he bumped into a guy ... and things escalated quickly.

We're told Mario said "excuse me" to the man, who replied "you got a problem?" The dude was about to step to Mario and make physical contact before an employee got between them.

Mario had left his phone on one of the locker room benches before all of this, and said he just wanted to get his phone and leave. That's when we're told the guy suddenly grabbed Mario's phone and threw it against a wall ... smashing it to pieces.

Security arrived and cops were eventually called. Law enforcement sources tell us they responded to the Planet Hollywood Spa, but no arrests were made as Mario refused to press charges. He did, however, get his phone replaced by the hotel free of charge. Later that night, Mario caught Jennifer Lopez's show with his wife ... so it seems like he's alright.

We're told the man that almost fought with Mario is a known troublemaker ... and is now banned from the facility. We've reached out to Mario's camp ... so far, no word back.