Importing Canadian aircraft into US

Steve Foley wrote:

[...] Canadian registered 206 and am importing it into the US.
The aircraft just had an annual, but the owner doesn't have
insurance on it, so the option of having him fly it to the US
isn't there.

Where is it written that insurance is required to fly in the US?

Unless the airplane is already sitting on the border, it would have to
fly some way within canadian airspace. *That* is forbidden without
insurance.

- FChE