On Saturday morning X17 photographers spotted Benji Madden and Cameron Diaz leaving her Hollywood home after a sleepover -- who would have guessed that these two were hooking up?!

A source tells X17online exclusively that the 41-year-old actress and the 35-year-old Good Charlotte rocker have been dating for about a month now, and get this -- Nicole Richie was the one who introduced the duo!

Our source continued:
    "Cameron is pretty into him ... she likes the bad boys! But the thing is, Benji's actually a really nice guy. They've been having fun together, making sure it doesn't get too serious too fast."

Just the other day, Cameron attended Gwyneth Paltrow's Goop party in Los Angeles, and Nicole and hubby Joel Madden were there, but we didn't see any pics of Cammy and her man. Sneaky! We're guessing it'll only be a matter of time before the Madden bros take their ladies out on a double date...

P.S. Can we call them Camji?