This guy, Kenneth Lamb, in a February 2008 blog post. And his source for the assertion? He didn't offer one. He said that Obama has only "only 1 relative ethnically African Negro - a maternal great-grandparent (Sen. Obama's great-great grandparent)" but he didn't offer any evidence for this.
In fact, it's rather impressive that Lamb managed to identify the race of, at least, Obama's eight African great-great-grandparents. Because he certainly didn't get that information from any public source. Rather, he openly admits that's his statements are based on "original research"; he just doesn't share any of his sources.
The closest he comes to offering a source for his claim in the entire post is when he says that his father was "his father was officially classified as "Arab African" by the Kenyan government." But he offers no links or citations or even names of what documents he's supposedly referring to.