Quintilian, and J.S. Watson. Institutes of Oratory.Edited by Lee Honeycutt, 2010. Kindle Electronic Edition. Book V Chapter 5.
Another source of evidence Quintilian identifies is that of documents (Quintilian V.5.1). Care must be used to prove the origin and truthfulness of the documents, as well as whether they actually prove what is alleged (Quintilian V.5.2).