Jimmy Fallon Shows Up at Jonas Brothers’ Concert and Sings ‘Mr. Brightside’ Onstage

“Nick asked him to come do it a week ago,” a source tells PEOPLE about Jimmy Fallon’s surprise rockstar moment

Jimmy Fallon is unleashing his inner rock star!

The Tonight Show host, 48, surprised concertgoers at The Jonas Brothers’ tour stop in New York City on Sunday by walking onstage to sing The Killers’ rock classic “Mr. Brightside.”

Fans posted videos on Twitter (now known as X) expressing how shocked they were to see Fallon appear at the band’s second sold-out show at Yankee Stadium, where their new run of dates titled The Tour kicked off Saturday.

Jimmy Fallon surprising fans onstage at The Jonas Brothers' Tour with a cover of Mr. Brightside by The Killers.

Jimmy Fallon.Jeffrey Tousey

“How would you all like to be in the world’s biggest karaoke party for two minutes?” Fallon asked the 60,000 crowd, according to one fan’s video.

“I hope you know the words, this is ‘Mr. Brightside,’” he added before the crowd energetically joined in with his performance.

Nick Jonas, Kevin Jonas, and Joe Jonas perform onstage during Jonas Brothers "Five Albums, One Night" Tour

Nick Jonas, Kevin Jonas and Joe Jonas perform onstage during Jonas Brothers “Five Albums, One Night” Tour.Kevin Mazur/Getty Images for Live Nation

“I’ve brought my boombox,” Fallon said in another fan video, which showed him gesturing to a cylinder bag he carried onstage with a cross-body strap.

THE TONIGHT SHOW STARRING JIMMY FALLON -- Episode 0972 -- Pictured: (l-r) Singer Dolly Parton and host Jimmy Fallon sing a "Holiday Song Medley" on November 30, 2018

Jimmy Fallon with Dolly Parton on The Tonight Show.Andrew Lipovsky/NBCU Photo Bank/NBCUniversal via Getty

“Nick asked him to come do it a week ago,” a source tells PEOPLE about the surprise musical moment. “He told him he could do whatever he wanted and Jimmy was like, ‘Anything — even karaoke with my boombox?!’ “

The source also confirmed that Fallon performed the song with “no rehearsal.”

“Jimmy Fallon just randomly showed up at Jonas Brothers Yankee Stadium Night 2 and is leading the crowd in “Mr. Brightside,’ ” a fan captioned a further video post.

The Jonas Brothers recently spoke exclusively to PEOPLE about their tour, calling it the “most ambitious show” they have ever attempted.

Promising to cover their entire history as a band, the trio of brothers — Joe, 33, Nick, 31, and Kevin, 35 — will put on 90 performances in 20 countries.

“If you’re an OG superfan or if you’re more into the new stuff, I think you’re going to be pleasantly surprised and have a great time,” Joe told PEOPLE. “There’s something for everybody.”