Proof would be a recording of him saying it. Evidence that would help remove some reasonable doubt would be a person coming forth with evidence they were around the president when he said it.... For example, if Kushner came forth and said he said it, it would give credibility...but as the statement stands, there is nothing to go on...For example I could write an article that says, "a source close to the administration says Trump stated we never landed on the moon"
To break this down 1) close to the administration is vague and is a reference term that doesn't mean anything. I could say that I met Trump once, and therefore I am close with the president, that's my opinion.
2) I could be making a lie that you can't verify. How can you verify that Trump said people didn't land on the moon?
Think of this as a court setting. I claim the defendant said something. I can't tell you who told me and I don't have any recording of it. How would a judge look at it?
I'm looking at this as being unbiased. Many posters here are so controlled by their political feelings and don't even realize it.