On Tuesday, January 29, 1946, M. Charles Dubost, a French Deputy Chief Prosecutor at the big Nuremberg trial, read from document RF-140:The first page of Document Number F-140 states and I quote so as not to have to return to it again-in the fourth paragraph which deals with Auschwitz: "About seven million persons died in this camp."
Trial of the Major War Criminals Before The International Military Tribunal Nuremberg 14 November 1945 — 1 October 1946. Volume VI, Proceedings 22 January — 4 February 1946. 1947. p.323.
Yale Law School's Avalon Project, online version