From the Back of the Book:

Subsequently, you have from the scholars those who have compiled forty Hadeeth concerning the fundamentals of the religion; others concerning the subsidiary branches, others concerning Jihaad, others concerning Zuhd, others concerning manners, and others concerning admonishments. All of them are righteous intentions and may Allaah be pleased with those who had these righteous intentions. I deemed it appropriate to compile forty (Hadeeth) more important than all of these entirely, and it is forty hadeeth that encompasses all of that. Every hadeeth has in it a grand principle from the principles of the religion.

The scholars have described it as Islaam orbits and revolves around it, or that it is half of Islaam, or a third of it, or similar to these (statements). Thereafter, I was devoted to making these forty (Hadeeth) authentic. The majority of it is in Bukhaaree and Muslim. I mentioned them without the chains of narration to facilitate its memorization and to make its benefit universal, by the will of Allaah, The Exalted. Then I trailed it with chapter headings that are precise in their wording.

It is appropriate for every person who is striving for the hereafter to have knowledge of these ahadeeth due to what they incorporate of important affairs and due to what they comprise of exhortation and directing to all of the obedient acts. That is apparent to anyone who reflects.