From the movie, the full quote is,
There's absolutely no reason for us to assume the Germans are mounting a major offensive. The weather is awful, their supplies are low, and the German Army hasn't mounted a winter offensive since the time of Frederick the Great. Therefore, I believe that's exactly what they're going to do.
I have not been able to find any primary source for this quote.
Eisenhower and Bradley knew their lines were overextended in Ardennes. The US Army had had more casualties than expected, and less replacements. Ike's broad front strategy also involved keeping pressure at least on small areas, so it required some concentration of troops to attack.
Bradley explicitly stated that the VIII Corps was deliberately thinned to thicken the winter offensive and that even if the germans were to bust all the way to the Meuse, nothing in the Ardennes would make it worth his while. Bradley even conducted a wargame to Ike about it, and coarsely described the limits of the possible german advance (which proved to be relatively accurate)
==> For various reasons, they decided to take the risk.
12/16, the offensive starts. Ike and Bradley are away (Ike was getting his 5th star), and were briefed on the evening. Bradley thought it was an spoiling attack, a diversion to stop his own attack, but Ike overruled him, sending two divisions from neighboring armies to help, and in the next day (12/17), sending their only reserve, the two airborne divisions. On the 17, he also asked Patton about how he would be able to assist.
The first highest-level meeting was on 12/19 Ike, Bradley, Patton, Devers, Smith, Tedder. Monty sent Maj. Gen. Freddy Guingand.
They decided to counter attack, Ike stated they should take the offensive as an opportunity to destroy german divisions. Ike drew the line at the Meuse. Ike asked Patton when he could attack, at least with 6 divisions.
Patton astonished them telling "3 divisions on 12/21". Ike didn't believe him, was irritated and responded "Don't be fatuous, George". But it was true, after the meeting Patton called his HQ and just gave a code word to start the offensive. And his army pivoted north very fast.
This only was possible because Patton started detailed planning right on the 16th.
Patton had two aides who he praised much for being smart and diligent: Cel. Koch, intelligence, and his logistic planning man (I don't remember the name, he is cited in another source).
Koch had briefed Patton on 12/09 about a concentration of german units opposite to the Ardennes. He also counted 8 german armored divisions which were not where they were supposed to be, plus parachute and Panzergrenadier units. Patton had the foresight to order him to collect all available information, even outside of the area covered by their own army. The VIII Corps also reported unusual enemy concentrations in early December.
Patton probably was brainstorming options or even making less detailed plans since that time. We know Patton knew about Ike's and Bradley's concerns about the Ardennes because Patton noted in his diary, late November, that Bradley was making a mistake about leaving the VIII corps static in the Ardennes for so long.
In short: All the higher brass knew about the potential problem. Patton, having chosen immediate subordinates who he judged very capable, had the foresight of ordering them:
Thus, the quote does not appear as something Patton would say.
Source: The European Campaign: Its Origins and Conduct S. Newland; C. Chun; Strategic Studies Institute Ch. 8, The Ardennes Offensive
