Why did Hitler have such a disliking for Slavic people? and what he had written in "Mein Kampf" about them?

When Hitler’s armies invaded Poland in September 1939, they were under orders to carry out Hitler’s policy of obtaining Lebensraum, living space, for the German people. As Richard Lukas states: “To the Nazis, the Poles were Untermenschen (subhumans) who occupied a land which was part of the Lebensraum (living space) coveted by the superior German race.” Thus, Hitler authorized his troops to kill “without pity or mercy all men, women, and children of Polish descent or language. Only in this way can we obtain the living space we need.”