On Thursday night, Donald Trump was busy at a rally defending waterboarding — but the parents of U.S. Army Captain Humayun S. M. Khan were busy in Philadelphia sharing a different kind of message.

Khan was a U.S. Army captain and a Muslim who was killed in 2004 when a suicide bomber attacked his infantry unit. On the stage of the Democratic National Convention Thursday night, Khan's parents gave a speech that simultaneously honored their son's legacy and delivered a stinging rebuke to Donald Trump.

"Hillary Clinton was right when she called my son the best of America," Khan's father, Khizr Khan, said. "If it was up to Donald Trump, he would have never been in America. Donald Trump consistently smears the character of Muslims. He disrespects other minorities, women, judges, even his own party leadership."

Khizr Khan, whose son died serving in the US military, schools Donald Trump on the Constitution #DemsInPhilly pic.twitter.com/ElGMQTALcu — Mashable News (@MashableNews) July 29, 2016

Trump has called for a temporary ban on Muslims entering America. In November, he said he was open to the idea of requiring Muslim Americans to register, causing an uproar even in his own party.

But Khan's strongest moment came when he pulled out a copy of the United States Constitution.

Khizr Khan, father of fallen US Army Capt. Humayun S. M. Khan holds up a copy of the Constitution of the United States. Image: AP/J. Scott Applewhite

Easily the most powerful moment of DNC - parents of a U.S Army officer, a Muslim, who sacrified himself to save his Army buddies — West Wing Reports (@WestWingReport) July 29, 2016

Trump "loves to build walls and ban us from his country," Khan said. "Donald Trump, you are asking Americans to trust you with our future. Let me ask you, have you ever read the United States Constitution? I will gladly lend you my copy."

Judging by the subsequent roar of the convention crowd, in under ten minutes, Khan transformed his grief into one of the most powerful indictments of the convention so far.