The Interpretation of Dreams by Sigmund Freud Chapter 8 Page 27

the symptom formed that these normal thoughts have been subjected to abnormal treatment and have been transformed into the symptom by means of condensation and compromise formation, through superficial associations, under cover of contradictions, and eventually over the road of regression. In view of the complete identity found between the peculiarities of the dream-work and of the psychic activity forming the psychoneurotic symptoms, we shall feel justified in transferring to the dream the conclusions urged upon us by hysteria.

From the theory of hysteria we borrow the proposition that such an abnormal psychic elaboration of a normal train of thought takes place only when the latter has been used for the transference of an unconscious wish which dates from the infantile life and is in a state of repression.