That 90% number was from an interview he did with CNBC. He didn't call for it, but he didn't take it off the table either.
http://thinkprogress.org/economy/2015/05/26/3662773/sanders-90-percent-tax/
In
an interview with CNBC’s John Harwood, Sen. Bernie Sanders (I-VT), who is running for the Democratic presidential candidacy, said he could back a 90 percent top marginal tax rate.
Harwood brought up that some
have likened efforts to combat income inequality to Nazi Germany. Sanders noted sarcastically, “When radical, socialist Dwight D. Eisenhower was president, I think the highest marginal tax rate was something like 90 percent.”
Harwood followed up by asking, “When you think about something like 90 percent, you don’t think that’s obviously too high?” to which Sanders replied, “No.”