After the speech he looked at us ironically and added softly: "Otherwise you would die". The words were icy but the tone like velvet, almost friendly.
Source, in full
Benjamin Murmelstein, Theresienstadt: Eichmanns Vorzeige-Ghetto, ISBN 978-3707605105, as quoted in Eichmann's Jews: The Jewish Administration of Holocaust Vienna, 1938-1945 by Doron Rabinovici (2014), ISBN 978-0745646824.