There's a lot of articles that mentioned this comment. Here's one for example
http://blog.moviefone.com/2007/02/10/george-lucas-empire-strikes-back-was-the-worst-star-wars-film/
I'm sure you can google more. On the other hand, I recently watched the blu ray special about empire and Lucas didn't say anything negative about it. He seemed mostly to think it didn't have enough action or was too slow