NBC News, The Hill and Politico are all reporting today that "an Obama campaign official" told each of them the same thing: Vice President Joe Biden had his first practice debate session last week. It's also possible one publication had it first and the other two stole it, while pretending they had the news separately, but that's a different post for a different day.
It looks like The Hill had the news first (even if it is the exact same news and quote the other two have) so we'll go with them:
"The Vice President has been preparing for his debate with Congressman Ryan – a candidate who has an exceptional grasp of policy details and budgetary matters," the official said. "He and Congressman [Chris] Van Hollen have held their first mock debate session."
Van Hollen is serving as the stand-in for Paul Ryan in Biden's preparation.
The veep debate is scheduled for Oct. 11.