Whether he said it or not, he definitely practiced it. For many years, Feynman taught an informal seminar for freshmen at Caltech, called "Physics X", at which he offered to answer any physics questions the freshmen could come up with.
Not quite any physics questions. Feynman imposed a rule that he would "not take any question the answer to which did not match the impedance of your knowledge." So a freshman asking "What is quantum chromodynamics?" was gently nudged into a simpler answer that they might learn something from.
"If you can't explain something to a first year student, then you haven't really understood it."
Although WikiQuotes says it's unsourced: http://en.wikiquote.org/wiki/Talk:Richard_Feynman