The evidence is that Assange emphatically said it
wasn't Russia, flatly said it was someone internal to the DNC, and he's never been shown to be wrong or dishonest about Wikileaks business.
Additionally, Assange has implied it was Seth Rich as strongly as he can without actually confirming he was the source, because he's bound by journalistic ethics and his agreement with his source to not reveal them.
Lastly, the Mueller report tries to discredit this and states as fact that Seth Rich was not the source. But it provides absolutely zero reasoning or evidence for this, and there is literally no way they could know this for sure. It claims it as fact regardless, and so does the entire establishment corporate media.
