You see this quote accredited to Hitler with no source given.
Hitler wasn't that "quippy".
That is why he had a brilliant Minister of Propaganda, Joseph Goebbels.
Goebbels "quoted" all the time, it was his job ..... not for any particular reason.
I venture to say that it was Goebbels who came up with the quote as part of his job, but adopted by Hitler.
That is why you cannot locate a source as to where and when and what purpose Hitler came up with it ... he didn't.
Here is the ENTIRE Goebbels quote.
Joseph Goebbels quotes
"If you tell a lie big enough and keep repeating it, people will eventually come to believe it. The lie can be maintained only for such time as the State can shield the people from the political, economic and/or military consequences of the lie. It thus becomes vitally important for the State to use all of its powers to repress dissent, for the truth is the mortal enemy of the lie, and thus by extension, the truth is the greatest enemy of the State.”
Even this quote wasn't origional to Goebbels .
Several other famous indiviuals have quoted similar statements long before HItler and Goebbels.
"A lie told often enough becomes truth" - Vladimir Lenin.
"There's nothing so absurd that if you repeat it often enough, people will believe it." - William James (1842-1910) The father of modern Psychology