Warren Buffett tore into Republican presidential candidate Donald Trump at a campaign rally for Hillary Clinton on Monday, criticizing Trump’s war of words with the family of a Muslim American soldier who was killed in action.

In introducing Clinton at an event in the billionaire investor’s hometown of Omaha, Neb., Buffett took exception to Trump’s claim that he, like the family of Army Capt. Humayun Khan, had made sacrifices. “How in the world can you stand up to a couple of parents who lost a son and talk about sacrificing because you were building a bunch of buildings?” Buffett said. “Trump and his family have sacrificed nothing; building buildings is not sacrifice.”

Buffett to Trump: 'Have you no sense of decency?'

“I ask Donald Trump: ‘Have you no sense of decency, sir?’” Buffett said, echoing a famous quotation from the anti-communist Army-McCarthy hearings in the 1950s.

Read: Khizr Khan vs. Donald Trump: Whose Constitution will Americans choose?

Buffett, the founder of Berkshire Hathaway Inc. BRK.B, -2.37% BRK.A, -2.46% , also called on Trump to release his tax returns as all other major presidential candidates have done since 1980. Trump has refused to do, saying he cannot release his returns because he is being audited by the IRS.

Buffett said that he, too, is under audit by the IRS, and proposed meeting with Trump “any time, any place” to go over tax returns together.

“You’re only afraid if you’ve got something to be afraid about,” Buffett told the crowd. “He’s not afraid because of the IRS. He’s afraid because of you.”

Also read:Buffett baits Trump on taxes: Show me yours, and I’ll show you mine

Buffett went on to urge voters to turn out in November, and he pledged to personally drive people to the polls who otherwise would not vote.

Clinton then laid out her economic plan, pulling in parts of her nomination acceptance speech at last week’s Democratic National Convention. Clinton again knocked Trump for using overseas manufacturers for the many products that carry the Trump brand, called for higher taxes for the richest Americans and vowed to improve the nation’s infrastructure.

Buffett, 85, the fourth-richest man in the world, formally endorsed Clinton in December and is one of several prominent billionaires backing her candidacy. Those ranks also include former New York Mayor Michael Bloomberg, the founder of Bloomberg LP, and entrepreneur and Dallas Mavericks owner Mark Cuban.