The history pages of the Imperial Library say/said so (at some point). It isn?t in the Lore. Also, the battle against the Lefthanded Elves was in the Merethic Era, rather than in the seventh century of the First.
I have issues with that, since Yukoda didn't explode till 792, while Red Mountain was 92 years earlier. In addition, the first Yokudans to arrive in Tamriel were in 808, which means it had been 102 years after the dwemer disappeared from the world. Not to mention the dwemer were int he opposite direction of the Yukodans, so I don't know how they could have had a war, then suddenly live in two different continents in opposite directions from each other, unless one was severely driven away. Even if it was in the Merethic Era