“Nobody believes a rumor here (in Washington, DC—ed.) until it’s officially denied” was said by labor attorney Edward Cheyfitz (1913-1959) in 1958, recorded in the magazine Look. Rumors that don’t have substance are ignored; if a rumor is officially denied and put on the record, the thought is that the rumor must have had some truth to it. The Cheyfitz quotation appears in several anthologies.
 
 
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The Municipal Employee
1958
Pg. 42:
Attorney Edward Cheyfitz on Washington gossip: Nobody believes a rumor here until it’s officially denied.
—Irv Kupcinet, quoted in Look
 
25 May 1959, Hagerstown (MD) Daily Mail, pg. 4, col. 4:
WASHINGTON (AP)—Edward T. Cheyfitz, 45, a labor management consultant, died Sunday. He had suffered a massive coronary thrombosis last Monday.
 
30 December 1962, San Antonio (TX) Light, American Weekly, pg. 5, col. 2: 
A cynical but quite accurate comment on Washington gossip is only one of the reasons that the late Edward Cheyfitz, a prominent Washington attorney, will be remembered.
 
“Nobody believes a rumor here,” Cheyfitz said, “until it’s officially denied.”
 
Google Books
Speech Can Change Your Life:
Tips on speech, conversation, and speechmaking
By Dorothy Sarnoff
New York, NY: Doubleday
1970
Pg. 322:
Nobody believes a rumor here until it’s officially denied. — Edward Cheyfitz
 
22 September 1980, Dallas (TX) Morning News, “Who Said That?,” pg. 42:
Nobody believes a rumor here in Washington until it’s officially denied.—Edward Cheyfitz.