If you repeat a lie often enough, it becomes the truth.
If you repeat a lie often enough, people will believe it.
If you repeat a lie long enough, it becomes truth.
If you repeat a lie many times, people are bound to start believing it.
If you tell a lie big enough and keep repeating it, people will eventually come to believe it.
No reliable source; probably misquotations of the Big Lie idea.
Many sources can be found for this quote from before Goebbels came to power.