Beyonce Knowles picked a very special occasion for her first post-baby outing.

Four weeks after giving birth, the 16-time Grammy winner supported her husband Jay-Z at his Carnegie Hall concert in Manhattan, which benefited The United Way of New York City and his Shawn Carter Foundation.

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The new mom showed off her svelte body in a body-hugging Alice by Temperley dress with Ofira & Lorraine Schwartz jewelry, an Alexander McQueen clutch, and Christian Louboutin platform heels.

During the concert, Jay-Z got choked up while performing "Glory," the song he wrote for their daughter, Blue Ivy (born January 7 in NYC).

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"You could tell Jay is over the moon," a concertgoer tells Us Weekly. "He asked the crowd if there are any proud parents out there. Then after he said, 'I didn't think I was gonna make it through that one -- that was tough."

The rapper was later joined onstage by surprise performers Alicia Keys, Nas and Questlove.

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Post-show, A-list guests hit up Jay-Z's 40/40 club for the official after-party. Knowles was clearly enjoying her night off, arriving half an hour after her hip-hop husband, 42, walked the red carpet.

"She made a much bigger effort to say hello to nearly everyone, making rounds around the whole club before joining Jay-Z and guests in the VIP room," an onlooker tells Us. Neither Knowles nor Jay-Z (real name: Shawn Carter) spoke to reporters.

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Knowles opted not to imbibe at the after-party, instead sucking on lemon wedges throughout most of the evening. Other attendees included Brooklyn Decker, Melanie Fiona and Angie Martinez.