CNN talking heads, Wall Street Journal and NY Times writers, President Obama, either directly or through administration spokespeople. You don't need to be an expert. You just need to read the Ukraine Constitution. Since no one's challenged Obama's statement that the vote is illegal under Ukraine law it's reasonable to assume that that is a true statement. Do you have information to the contrary? If so please post it.
In any case I've already stated that if the Crimean people want to be part of Russia that should be allowed to happen.