“Truth is so hard to tell, it sometimes needs fiction to make it plausible.”

Source: The Six Rules of Maybe

Source: Dead as a Doornail

From Interviews

Source: The Storyteller

Why our future depends on libraries, reading and daydreaming (2013)
Context: We writers – and especially writers for children, but all writers – have an obligation to our readers: it's the obligation to write true things, especially important when we are creating tales of people who do not exist in places that never were – to understand that truth is not in what happens but what it tells us about who we are. Fiction is the lie that tells the truth, after all.

Pablo Picasso said something very similar. Perhaps it is the source? From Herschel B. Chipp’s Theories of Modern Art: "We all know that Art is not truth. Art is a lie that makes us realize truth, at least the truth that is given us to understand."
Disputed

Interview at the World Science Fiction Convention (25 June 1998) http://www.chicon.org/gohs/turtldov.htm