The great Greco-Roman historians regularly testified that their content was derived from 1st hand experience (their own or that of witnesses who they interrogated), or was at least grounded in witness-testimony (2nd hand, maybe 3rd).
For 5 simple examples and more:
This is relevant because these historians are representative of a the larger mass of historians and standard expectations.
Greco-Roman historians jumped quickest to record events they saw 1st hand. Compared to modern history, writings of ancient historians are disproportionately about things historians were themselves involved in.
Consider these 7 examples, and more…
This is relevant because they, by contrast, did not synthesize and write as much on all the other events they had ample documentary access too.
Historians in the 1st-2nd century would sometimes tip readers off to their witnesses by how they conspicuosly included the person in the account, especially at a key moment where a witness would be needed.
A full page at /greco-romans/histories/cite-witnesses-via-emphasis will analyze this evidence:
Greco-Roman historiographers tended to successfully ground their material in witness testimony or approval.
See this page to analyze 4 arguments:
This is relevant because if they did tend to successfully obtain witness testimony or close, then it is only natural that they would intimate to readers, implicitly or explicitly, that they did. (Rarely do we put such difficult work into things while leaving our audience completely in the dark that it was done; audiences will usually be aware one way or another.)