Superman / Batman #3

In the days before Batman and Superman worked together regularly, they both made plans to board a cruise to the Bahamas in their civilian identities, neither knowing that assassin Slade Wilson was following to kill Bruce Wayne. But before Wilson can kill Wayne, the ship encounters a spatial rift, bringing with it the Crime Syndicate and a character intent on defending Bruce Wayne. Superman and Batman send the inter-dimensional travelers back to their universe and subdue Wilson, never knowing who was behind it all or why.