Batman #613, "Hush, Chapter Six: The Opera"
Bruce attends the opera with Selena Kyle, Leslie Thompkins, and Tommy Elliot, but Harley Quinn crashes the show. Bruce manages to slip away to don his suit, and a showdown between Batman, Catwoman and Harley goes down. Batman is a little surprised by the excess violence demonstrated by Harley, making him suspect that the same hand that has been driving Killer Croc and Poison Ivy is behind this as well.