George Clooney and Amal Alamuddin toast engagement at Bono's star-studded birthday party

and

arrived hand-in-hand for a star-studded party in Malibu on Sunday to

. The happy couple were pictured arriving at Cafe Habana, where they joined the likes of

, her husband

, who owns the restaurant, and U2's

.The celebrity group gathered on the back patio, where they

.

© Photo: Splash

George Clooney and fiancée Amal Alamuddin were spotted together at Bono's birthday bash

"

" a source told

. "

."Smiling Amal could be seen

as she chatted with her husband-to-be's close friends.There was even a bit of karaoke;

, who reportedly duetted with her daughter Kaia.

© Photo: Splash

Rande Gerber and Cindy Crawford were also seen at the party

An eyewitness told

that the venue was busy with people celebrating Mother's Day. "They got a show they didn't expect and

."George, 53,

to British human rights lawyer Amal, 36, in late April, after seven months of dating.The future Mr and Mrs Clooney recently returned from a holiday in Cabo, Mexico, where they celebrated a friend's wedding. "You can tell these two are in love," an onlooker said.