I think you'd be hard pressed to find such a law. What you will more likely find that it is illegal for an employer to discriminate in hiring based on that information, and an employer asking the question in an interview is quite likely to be used in any lawsuit over alleged discrimination as evidence that they were, in fact, discriminating in hiring based on the information, since asking a question in an hiring interview is pretty compelling evidence of an intent to use the response in making hiring decisions.