Anything in the Gospels is "Double-Hearsay", defined as the following (from Wikipedia)
Double hearsay is a hearsay statement that contains another hearsay statement itself.
For example, a witness wants to testify that "a very reliable man informed me that Wools-Sampson told him." The statements of the
very reliable man and
Wools-Sampson are both hearsay submissions on the part of the witness, and the second hearsay (the statement of
Wools-Sampson) depends on the first (the statement of the
very reliable man). In a court, both layers of hearsay must be found separately admissible. In this example, the first hearsay also comes from an
anonymous source, and the admissibility of an anonymous statement requires additional
legal burden of proof.
I think that the only way that one could legitimately admit any of the Gospels as evidence would be to assume (erroneously) that the Gospels were written by the men they were named for. In the early 1800s, I think this would have been a common mistake. Now, it is a view not held by most Secular Biblical Scholars (unbiased scholars). That is why I think there would be a specific issue with any of Jesus' supposed quotes.