ABBA celebrated their 50th anniversary at a private party in Stockholm last night. During the show, for a brief, historic moment, Björn Ulvaeus and Benny Andersson (who met for the first time 50 years ago in June 1966) joined Agnetha Fältskog and Frida Lyngstad to perform together on stage, according to Expressen (via Billboard) and Eurovision director Edward Af Sillén. Back in January, the group shared a stage in Stockholm for the opening of Mamma Mia! The Party, but didn't perform. This time, Expressen reports that Fältskog and Lyngstad played “You & I,” apparently referring to “The Way Old Friends Do,” before Ulvaeus and Andersson joined them. Though video is yet to emerge, you can see pictures from the night below.