Freud refers to a scene in Act II of La Belle Hélène, by German/French composer Jacques Offenbach (1819-1880). Helen of Troy, finding Paris in her bedchamber, provides him with the perfect excuse for his planned adultery by exclaiming that "Ce n'est qu'un rêve" (It is only a dream).
La Belle Hélène by Jacques Offenbach
Clip from a production at the Paris Théâtre du Châtelet, starring Felicity Lott and Yann Beuron, (2001)
Online text of the libretto, by Henri Meilhac and Ludovic Halévy, (first performed 1864)