I've never seen any direct quotes from Lucas in which he said that the Falcon was based off of a burger. But I've seen the story perpetuated over and over again, as if he had (as with so many other half-truths and non-truths; sourcing claims is not something people are big on).

So let's take it from the top; Dale Pollock, in Skywalking page 210:

And who else [but Lucas] would refer to the *Falcon* as “a hamburger-shaped spaceship”?

Doesn't indicate anything, except that the Falcon is somewhat hamburger-shaped. Says nothing about the design process. And then from the previous quote:

[...] whose redesign by Joe Johnston was purportedly inspired by Lucas’ idea of a hamburger in space.

Which could just as easily have come out of Johnston saying something along the lines of: "You know, George said just the other day, that the Falcon looked almost like a burger flying through space," and the writer not being careful enough with his choice of words, much like the 'Slave 1 was based on a streetlamp'-myth (untrue).