I have answered this question before here Also, stayed tuned tomorrow when there will be an AH podcast coming out with me talking about this subject.

In Mein Kampf, Hitler said,

"If 12-15,000 'Hebrew corrupters of the people' had been held under poison gas at the start of the First World War, then 'the sacrifice of millions at the front would not have been in vain'.

Hitler also stated in a speech in 1939 that

"Today I will once more be a prophet: If the international Jewish financiers in and outside Europe should succeed in plunging the nations once more into a world war, then the result will not be the bolshevization of the earth, and thus the victory of Jewry, but the annihilation of the Jewish race in Europe!"

These are the two primary statements supporting it. There is, however, a much more in depth discussion. Gerald Fleming believes that Hitler "continusously held this goal [extermination of the Jews] up before the German nation." He argues that the statements made by Hitler and his associates form a continual escalation of the hatred toward the Jews. That this escalation was intentional as a set of "steps" toward genocide.

Sources Used: Niewyk, Donald. Problems in European Civilization: The Holocaust 2nd Edition. Boston: Houghton Mifflin, 1997.

Fleming, Gerald. Hitler and the Final Solution. Berkley: University of California Press, 1987.