It is real and it was used at the Nuremberg Party Rally in September 1933 after the Nazis had taken power. Being a sort of revue of the Nazi rise to power, Hitler said:

And so, I established in 1919 a program and tendency that was a conscious slap in the face of the democratic-pacifist world (…) [We knew] it might take five or ten or twenty years, yet gradually an authoritarian state arose within the democratic state, and a nucleus of fanatical devotion and ruthless determination formed in a wretched world that lacked basic convictions.

Only one danger could have jeopardized this development – if our adversaries had understood its principle, established a clear understanding of our ideas, and not offered any resistance. Or, alternatively, if they had from the first day annihilated with the utmost brutality the nucleus of our new movement.

Neither was done. The times were such that our adversaries were no longer capable of accomplishing our annihilation, nor did they have the nerve. Arguably, they furthermore lacked the understanding to assume a wholly appropriate attitude. Instead, they began to tyrannize our young movement by bourgeois means, and, by doing so, they assisted the process of natural selection in a very fortunate manner. From there on, it was only a question of time until the leadership of the nation would fall to our hardened human material. (…)

The more our adversaries believe they can obstruct our development by employing a degree of terror that is characteristic of their nature, the more they encourage it. Nietzsche said that a blow which does not kill a strong man only makes him stronger, and his words are confirmed a thousand times. Every blow strengthens our defiance, every persecution reinforces our single-minded determination, and the elements that do fall are good riddance to the movement.

As you can see, this was said, albeit there is vital context to the quotation making it clear that after a certain point, Hitler believed, the violence and resistance leveled against the Nazis proved useful in as far as – typical for the Social Darwinist thinking of the Nazis – it weeded out the weak among them and made them stronger.

Both his points are rather peculiar interpretations of the Nazis' own history and without going into counter-factual or speculative history, there indeed was a period in the NSDAP's history when it looked like as if the party would go the way of other völkisch movements in Germany and would simply vanish into obscurity.

Following the attempted coup d'etat in Bavaria by Hitler and his followers, Hitler was jailed – though with a very light sentence – and the NSDAP was prohibited as a party from November 1923 to February 1925 with all their assets seized and the party newspaper, Der Völkisch Beobacther, being outlawed. While the party existed forth under different names, it splintered and even the return of Hitler, who sought to manufacture a united right völkisch front could not prevent them from performing very badly at the urns and internal power struggles between various figures who advocated for various directions.

What brought them back from this was the economic crisis of 1929, Alfred Hugenburg's newspaper cartel, which disseminated positive reports about the NSDAP, and their use of communism as a boogeyman as well as later on, the massive support of the industrialist and conservative establishment. In 1930 the NSDAP garnered 18 percent of the vote in the elections but managed to in two years garner up to 37% in the last free elections in the Weimar Republic. This had partly to do with massive amounts of money they received from industrialists like Krupp and Thyssen, enabling them to have one of the most expensive elections campaigns up to this point that included all the bells and whistles at the time, from microphone cars to Hitler flying from one city in Germany to another (this was a huge thing at the time).

There are two dynamics here that are important: One is that the right and conservative elite were willing to embrace the Nazis because of their fear of communism and the second is that the left both in form of the communists and the SPD were expecting the Nazis to attempt another coup d'etat, which they then would resist and position themselves as saviors of the Republic. They didn't see it coming that the Nazis would gain power with the help of conservatives like von Papen, Schleicher, and Hindenburg (whom the SPD had supported over Hitler in the last German presidential elections).

Historically, the SPD had defeated the Kapp coup in the beginning of the 1920s via a general strike so there was some merit to the thinking that they could have defeated another coup attempt from the right. But they weren't prepared for this kind of take-over of power. Additionally, once comfortably in power, the call for resistance had the potential to end very badly, as the experience in 1934 Austria showed, where workers fought the fascist government and were crushed by the army.

In the end, the fear to employ what means they had at their disposal, including resistance, lead to paralysis on the left and as history has shown, for many of them to gruesome deaths in Concentration Camps. This, of course, lead to and still leads to severe debates among people opposed to Fascism, what an effective strategy against them is and while because of the contextual historical differences, there is little in the way of a golden strategy that we can glean from history, both the willingness to portray them as what they are, to deny them public space, and at some point to go all out as with the general strike against the Kapp coup have historically seemed to be viable options.