Dude, he says he “heard it from someone.” That’s it. That’s his source. From the link that directly quotes him:
“Before we continue with the story, let’s unpack Hersh’s part in this. Contacted by The Nation via e-mail, Hersh affirmed that he didn’t know that Butowsky had recorded the call, for which both the text and audio have been made available. Interviewed by NPR for its story, he admitted talking to Butowsky. “I hear gossip,” Hersh told NPR’s David Folkenflik. “[Butowsky] took two and two and made forty-five out of it.” Indeed, Butowsky, focused on coming up with an alternate explanation for the release of the DNC e-mails, and did go a lot further. But Hersh appears to have provided Butowsky (and by extension, Zimmerman and Wheeler) with unsupported claims about an FBI report on Rich—coming from a single unnamed source, from documents that Hersh admits he didn’t see but only heard about—that concluded that Rich was involved with WikiLeaks. As Caitlin Johnstone, writing for Medium, notes, “Hersh owes the world an explanation for his Seth Rich comments.”
Anyway. You still haven’t answered my question about Obama. I guess you won’t.