I would like to see proof of this
Well first of all, I thought it was actually common knowledge. I mean you can look up the numbers if you want (I actually did but I'm not going to share them because I don't care and it's not even a valid argument, see below), but I think if you thought about it you would see the truth of it. How many buildings do you know where blown up by Muslims? Two? Now Christians? What about the dozens of abortion clinics they blew up during the 80s, the Oklahoma City Bombing, etc?
In any case, it doesn't matter because my statement is not even a fair statement. If someone had said this to me in an argument I would have immediately attacked it, there's so many blatant errors in it. I mean, what do you consider a fair metric for religious terrorism in the United States? Number of buildings blown up? Value of damage? Number of people killed? Number of incidents? Depending on what you use you will get a different number and for different reasons they can all be considered a fair measurement. The conclusion that one religion has more violent tendencies is not a fair conclusion that can be made from this data in any case. How do you measure how much a religion caused you to do something? There are a wide variety of emotions, desires, and motives that are all at play when these people decide to do what they do. Then you have to decide if the religion is really that religion. I mean, I'm sure lots of Christians would love to brush off anyone who commits crimes in the name of their religion as "a member of a dangerous cult and not really Christian." It's not black and white.
Finally, I didn't even make an argument. I presented a statement and didn't even try to make a point with it. You may have read something into it, but I sure didn't say anything. My first question would have been "and what's your point," not "where's the proof?" Another example of an equally meaningful statement I could have made is "Buddhists get more traffic tickets than Hindus." If I had gone on to say "Buddhists are bad drivers" then I would be wrong, see the second paragraph. It's a sweeping generalization from a metric that doesn't really measure what you think it does.
Fun fact: Christian terrorists have blown up more buildings in this nation than Muslim terrorists have.
My goal here is not to argue that Christians are a bigger terrorist threat than Muslims in America. My goal here is to demonstrate that sort of an argument is ridiculous. The threat is not Christianity or Islam. The threat is terrorism.