From the Oak Leaves, Hemingway's Hometown Newspaper:
It’s a very compact, Hemingway-esque line, repeated often here in Oak Park. But it appears that Ernest Hemingway never said — or wrote — that his hometown was a place of “wide lawns and narrow minds.”
“We’ve never found it,” says Rose Marie Burwell, scholar and author of “Hemingway: The Postwar Years and the Posthumous Novels.”
However, Burwell thinks the line expresses what Hemingway saw as “the narrowness of the social and religious teachings that were so dense in his home.” Oftentimes, his frustration seemed to be with his family, rather than Oak Park itself.
from Robert K. Elder, editor in chief of the Oak Leaves