Hi, Genie-
The Wikipedia article on Woody Guthrie cites it, as do many other Websites - including this interesting blog. I haven't found anything close to an "original source," but here's the text from Wikipedia:

"This song is Copyrighted in U.S., under Seal of Copyright #154085, for a period of 28 years, and anybody caught singin' it without our permission, will be mighty good friends of ourn, cause we don't give a dern. Publish it. Write it. Sing it. Swing to it. Yodel it. We wrote it, that's all we wanted to do."
Written by Guthrie in the late 1930s on a songbook distributed to listeners of his L.A. radio show "Woody and Lefty Lou" who wanted the words to his recordings.

As its source for the quote, the Wikipedia article cites tulsaworld.com. I have an Oak Publications reprint of one of those mimeographed songbooks, titled American Folksong: Woody Guthrie, but it doesn't have Woody's copyright notice.

-Joe-

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There is an extensive discussion of Woody's views on copyright here (click). It's probably better to continue the Woody Copyright discussion in that other thread.