It feels a bit lame to post this, but since this is getting upvotes/interest... According to the Snopes article linked by Daniel in a comment:
this quotation appears to have been virtually non-existent until 2014, when it was used as the heading to Chapter 11 of Willfully Ignorant, a novel by Pat Miller set in Nazi-era Germany. (That book also mistakenly cited Mein Kampf as the source of the quotation.)
[...] it’s also false if simply considered as a shorthand version of what Hitler thought and/or did. These words reflect a misperception that Hitler’s political maneuvers on his way to seizing dictatorial control over Germany were akin to the legend of the boiled frog, that Hitler supposedly made a series of small, gradual changes in German law and government that chipped away at the rights of the citizenry over an extended period of time, avoiding any single large changes that might provoke resistance from the people until it was too late for them to effectively oppose him.
In fact, this is the historical opposite of what happened. Once he gained a position of political authority in Germany, Hitler quickly headed down the road of consolidating power by making very large changes in a short period of time. [...] The combination of the Reichstag Fire Decree and the Enabling Act effectively transformed Hitler’s government into a legal dictatorship within two months of his appointment as chancellor. These actions were not “imperceptible reductions” in freedom and rights — they were a massive change in the established order implemented in a very short period of time. Within another few weeks, Jewish businesses were openly attacked by the SA (the Nazi Party’s paramilitary organization, commonly known as “brown shirts” or “storm troopers”) and a “Law for the Restoration of the Professional Civil Service” was passed that banned all Jews, non-Germans, and political opponents from public service. A month later, trade unions were banned. Within another few months, all political parties other than the Nazi Party were banned. All of this took place within the six-month period after Hitler was appointed to the chancellor’s office, not “imperceptibly.”
Politifact also rates it as fake:
There is no record of Hitler saying or writing this statement, and it couldn’t be found in "Mein Kampf," his infamous autobiography and manifesto.
We traced the quotation back to the 2014 novel "Willfully Ignorant," by Pat Miller, who served as a Republican state legislator in Colorado in the early 1990s. Miller’s book uses the quote as a header at the start of a chapter and also falsely cites it as coming from Hitler’s "Mein Kampf".
We reached out to Miller about the quote but did not receive a response.
Besides there being no evidence that Hitler said or wrote these words, in "Mein Kampf" or otherwise, it also represents the opposite of his actions. [...]
And latter part is similar to Snopes'.
As to the actual origin of this quote, some discussion on wikiquote:
I first found the "quote" at the Association of British Drivers' website back in 2004, where it was attributed to Adolf Hitler but not specifically Mein Kampf. Nowadays the organisation is known as the Alliance of British Drivers and they've removed it from the website. I think the "taking away freedoms a little at a time" is mainly a modern negative side-effect of well-intentioned risk averse attitudes, typically involving legislating repeatedly for a small but dangerous minority of offenders. It is unlikely that Hitler would even have considered the "a little at a time" idea in his long-term plans. --Tws45 (talk) 23:47, 16 July 2016 (UTC)
Indeed, here it is, at the legacy site: http://original.abd.org.uk/resources/quotes/politics.htm (second one down on the right). --Tws45 (talk) 23:58, 16 July 2016 (UTC)
Also from the same website, but with a date attribution of 1936. http://original.abd.org.uk/downloads/otr/ABD_OTR_80.pdf Text is on page 12.
The direct link to pdf is dead now, and it's not archive.org either, but the other web link indeed (still has the quote). (The ABD is a bit more political than its naming might sugges.t)
