I recently pointed out that a December 1942 entry of Abraham Lewin’s Warsaw ghetto diary mentions the figure of six million Jews killed by Hitler. It turns out that Lewin was not the first Warsaw diarist to mention this figure. Earlier that month (December 14), Emmanuel Ringelblum recorded the following in his diary:
They have liquidated 5-6 million European Jews and it doesn’t seem that the German war machine suffers because of this.
Source: E. Ringelblum, Notatki z getta, Biuletyn Żydowskiego Instytutu Historycznego, No. 13-14, 1955, p. 266.