Justin Bieber had Beliebers in a tizzy when he performed the song that put him on the map.
"Reunited Rockin wit the Biebs. Surprised ATL tonight. #showkillas," the rapper proudly Instagrammed on Tuesday night.
Posting another photo of himself and Bieber onstage, he touted: "And the crowd goes wild...."
Beliebers did in fact go wild. "JUSTIN BROUGHT OUT LUDACRIS DURING BABY AND I'M CRYING BECAUSE I'M SO HAPPY BECAUSE IT BROUGHT BACK MEMORIES," one Twitter user posted along with a video of the performance.
"Ludacris and Justin singing 'Baby,' oh my god, yes," another concertgoer tweeted.
Bieber has certainly grown up in the six years since the release of "Baby." During his Atlanta show, he showcased his muscles and multiple tattoos in a tank top with Tupac Shakur's face on it -- a far cry from the purple hoodie and floppy haircut he sported back then.
But after all these years, he's still singing "Baby"
-- and so are his siblings. Check out Jazmyn and Jaxon's rendition of their older brother's hit song: