Presumptive Republican presidential nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE had a difficult time citing his sources when claiming Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE’s private email server had been hacked.

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In an interview with NBC’s Lester Holt set to air Thursday on “NBC Nightly News,” Trump was pressed to back up his claim that the foreign governments had breached the former secretary of State’s server.

“What evidence do you have?” Holt asked Trump.

Trump answered by saying Clinton shouldn’t have had a personal server because “it’s illegal.”

“But is there any evidence that it was hacked other than routine phishing?” Holt asked.

“I think I read that ... and I heard it,” Trump responded.

“Where?” Holt asked.

“I will report back to you,” Trump answered.

Trump has frequently slammed his Democratic rival for her use of a private email server while secretary of State, most recently on Wednesday in a speech denouncing Clinton’s qualifications.

“[She’s] a world-class liar,” Trump said in his 40-minute address in New York.