Looked it up, it's at the beginning of Maus II: And Here My Troubles Began, and goes like this:
"Mickey Mouse is the most miserable ideal ever revealed...Healthy emotions tell every independent young man and every honorable youth that the dirty and filth-covered vermin, the greatest bacteria carrier in the animal kingdom, cannot be the ideal type of animal...Away with Jewish brutalization of the people! Down with Mickey Mouse! Wear the Swastika Cross! --newspaper article, pomerania, Germany, mid-1930s"