Forbes and Fortune, both of which are cited by Wikipedia. A commenter further down questions their methodology.

Honest question - can anyone refute the claim that it is currently not the point in time when the largest share of wealth is in the smallest number of hands?

That source says that Rockefeller's net worth was $30 billion in current (2014) dollars.

The $340 billion figure is from a semi-nonsensical calculation that is popular because it yields eye-popping large numbers. They figured Rockefeller's assets equaled 1.5% of the US GDP at the time. And 1.5% of the current GDP would be about $340 billion dollars. But assets as a fraction of GDP is not very meaningful. It's like saying if I moved to a country with a small GDP such as Samoa I'd be richer than Bill Gates.

In effect, the $340B Rockefeller number is big because the US economy has grown. The worst part is many websites often confuse the scaled-GDP numbers with constant-dollar numbers, resulting in much confusion.