Joel Madden and Nicole Richie Joel Madden and Nicole Richie

Longtime couple Nicole Richie and Joel Madden are engaged, People reports.

Richie, 28, showed off her ring and announced the engagement during a Monday taping of The Late Show with David Letterman, a source told the magazine. The episode is scheduled to air Friday, but Madden, 30, couldn't keep the news to himself for long.

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"Yep. I'm engaged. Very happy. Yeah we've been engaged for a while so [you're] all [kinds] of late on that. But Thanks for the hooplah all the same," he tweeted Monday, thanking his family and friends for not letting the news slip.He later deleted the tweets — which can still be seen here

— and wrote Tuesday morning, "Just woke up. I'm screwed. Oh boy."Calls and e-mails to Richie's rep were not immediately returned.

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Together since 2006, Richie and Madden have two children together, 2-year-old daughter Harlow and 5-month-old son Sparrow.