Warren Buffett warned of the perils of excessive tariffs, saying they should not be a "weapon" and that the U.S. would be more prosperous if other countries were.

Before announcing his plan to step down as Berkshire Hathaway chief executive at the end of the year, Warren Buffett offered a fervent defense of trade on Saturday, saying tariffs should not be a "weapon" and the United States would be better off if other countries shared its prosperity.

"You're losing one of the most esteemed investors one could argue of all time," a source said of investors' reaction to Buffett's plan to step down.