From this:

In a chilling video on YouTube, Larry Grathwohl, a former member of the Weather Underground which Ayers helped found, says the organization planned to take over the U.S. government and give parts of the country to Russia, Cuba, North Vietnam, and China. The plans included "re-educating" Americans as Marxist-Leninist revolutionaries.

In the video, Grathwohl says he asked 25 leaders of the Weather Underground who were discussing the plans, "Well, what is going to happen to those people that we can't re-educate, that are die-hard capitalists?"

The reply, says Grathwohl, was that they would "have to be eliminated." When he pursued the question further, they estimated that "they'd have to eliminate 25 million people in these re-education centers. And when I say eliminate, I mean kill 25 million people."

Grathwohl was apparently an FBI informant who infiltrated the group, so take it for what it's worth. The exact video referred to isn't clear, but this might be it. His quotes were apparently featured in a recent Sean Hannity special; see this or the other videos from the same user. A documentary interview with Grathwohl is here. An unsourced (of course) "primer" is here, with more here.