How would you like to make $765 million on a Friday afternoon without moving a muscle?

Not so easy, unless you're Warren Buffett.

Shares in American Express surged Friday afternoon after a report that an activist investor had taken a stake in the financial services firm and wanted to push for changes.

AmEx shares spiked 6.7 percent on the news, making a tidy paper gain for the Oracle of Omaha.

Buffett's Berkshire Hathaway holds 151.6 million shares of the company, leading to a gain of $765 million as of 2:40 p.m. ET.

Making or losing a bunch of money in one day on a stock move is nothing new for Buffett, of course. Last month, he lost about $700 million in one day on IBM's post-earnings move.