Diodorus on his Sources for Egyptian History (based on Oldfather tr.)

Concerning the Historians, I must distinguish among them, to the effect that many have composed works on both Egypt and Ethiopia, of whom some have given credence to false report and others have invented many tales of their out of their own minds for the delectation of their readers, and so may justly be mistrusted. For example, Agatharchides of Cnidus in the 2nd Book of his work on Asia, and the compiler of geographies, Aretemidorus of Ephesus, in his 8th book, and certain others whose homes were in Egypt have recounted most of what I have set forth above and are, on the whole, accurate in all they have written. Moreover I myself, during the time of my visit to Egypt, associated with many of its priests and conversed as well with a number of ambassadors from Ethiopia who were then in Egypt; and after inquiring carefully from them about each matter and testing the stories of the historians, I have composed my account to accord with the opinions on which they most fully agree. (3.11)

Commentary (by D. Silverman)
Diodorus did indeed visit Egypt. But what he affirms here is antithetical to the historical method as formulated by Thucydides. Diodorus says he passed on the most widely accepted account; in other words, he did not apply critical acumen. The reader may note that Diodorus makes these claims immediately after he has recounted an especially outrageous tale about snakes who attack and eat elephants (3.10).