A message posted (not by me) through the PsyArt list:
Finally, Freud's quotation is found!
"I HAVE NO INCLINATION TO KEEP THE DOMAIN OF THE PSYCHOLOGICAL FLOATING AS IT WERE IN THE AIR, WITHOUT ANY ORGANIC FOUNDATION. LET THE BIOLOGISTS GO AS FAR AS THEY CAN AND LET US GO AS FAR AS WE CAN. SOME DAY THE TWO WILL MEET."
I have been working on this enigma for the past several weeks and going on a hunch, I have solved it.
The quote is from his paper:
"Reflections on parallel lines meeting; id-external reality intersections"
It was published in 1943 in the multidisciplinary journal, "Infinity and the Libido", volume 1.
—This would seem to be Freud's own notion of the future consilience of neurology and psychoanalysis.