There is no singular source.
Very quickly yesterday evening, in a matter of minutes, I found quotes from Augustine, Victorinius, and John Chrysostom showing that they believed the identity of Antichrist/man of sin and the "falling away" to be future events. Since they were writing in the 2nd, 3rd, and 4th centuries, we can pretty well establish that no one had clued them in that all was fulfilled in 70 AD.
Preterism is a house built on sand.
Would you argue that Victorinus' commentary on Revelation in general is accurate?
It's interesting that those who hold to an eschatology that can be verified over time, as we more farther and farther into the future, are so concerned about preterism. All you have to do is play a waiting game. If futurism is true then events will play out as you expect. No?
What I suspect is happening is that nervousness is setting in within the futurist camp. With every new book published and every new TV program about current events and Bible prophecy that proves to be wrong, the "faithful" of futurism become disillusioned and move to other, more biblically satisfying schools of eschatology. In some cases the sometimes connection between futurism and deficient soteriologies is recognized, and these folks move fully into the reformed/covenantal camp.
As I said, time is on someone's side. As a postmil I see the triumph of Christ and His truth in history. Futurism must give way.