Easy reading is damn hard writing.

Nathaniel Hawthorne (1804-1864) American writer

(Attributed)

Also attributed to others, including Ernest Hemingway. The reference to Hawthorne can be dated back to Maya Angelou in "The Art of Fiction," Paris Review, #116, Interview with George Plimpton (1990):

Nathaniel Hawthorne says, "Easy reading means damn hard writing."

Per Wikiquote, Angelou put it differently previously, in Conversations With Maya Angelou (1989) [ed. Jeffrey M. Elliot]:

I think it's Alexander Pope who says, "Easy writing is damn hard reading," and vice versa, easy reading is damn hard writing.

Which first clause may refer in turn not to Pope but Richard Brinsley, Clio's Protest, or the Picture Varnished (1771, pub. 1819):

You write with ease, to show your breeding, But easy writing's curst hard reading.