— Walt Disney American film producer and businessman 1901 - 1966
Tom Fitzgerald, a Disney Imagineer, as quoted in Ask Dave by Dave Smith (27 February - 12 March 2013) https://d23.com/d23-presents-ask-dave-answers-to-questions-asked/:
:: Despite its frequent publication, that is not a Walt Disney quote. We checked with Imagineer Tom Fitzgerald for the definitive answer: "I am very familiar with that line because I wrote it! It was written specifically for the Horizons attraction at Epcot and used in numerous ways, from dialogue in the ride to graphics. I find it amusing that the Science of Imagineering DVD series attributes it to Walt Disney, but I guess I should be flattered."
::* Disney Trivia from the Vault (2012) by Dave Smith, p. 243
Variant: If you can dream it, you can do it.