A line closely associated with Casablanca—"Play it again, Sam"—is not spoken in the film. When Ilsa first enters the Café Américain, she spots Sam and asks him, "Play it once, Sam, for old times' sake." After he feigns ignorance, she responds, "Play it, Sam. Play 'As Time Goes By'." Later that night, alone with Sam, Rick says, "You played it for her, you can play it for me", and "If she can stand it, I can! Play it!"

Source: Wikipedia