Where can I get factual linguistic information about rahman and rahim?

Rahman and rahim are both names of Allah, and frequently used. Now I once heard in this video that: Ar-Rahman = merciful to all Ar-Rahim = merciful to beleivers I fail to understand how this interpretation has come to exist. Though he explains syntax, he jumps to interpretation without reasoning, or at least I don't get it. Thus I searched and I couldn't find much linguistically presented analysis for the differences. For example, this video . Where can I find factual linguistics data, and not opinions?