Curiously, no. I mean, that's more or less the backstory that everybody accepts, but I don't think any episode or movie has stated it outright. It's just as possible that the Romulans were an exceptionally peaceful faction that escaped Vulcan when the remaining Vulcans were at their most violent, a bit before Surak's time. Or that the Romulans were the victims of a racist purge where all the ridgeheaded inhabitants of the planet, plus a few of their flathead supporters, were either slaughtered or driven off the planet.
The movie might answer this, you know.