Umm al-kitab (the basis of the book; the opening surah)

Umm al-shay' is the basis of a thing, and umm al-kitab is the basis of the Book. The Qur'an uses it for the firm verses to which one returns when there is doubt, and for the Preserved Tablet that holds God's knowledge. ¹

The hadith use it for the opening surah, as in the saying that whoever recites the umm al-kitab has been sufficed, and that a prayer without its recitation is deficient. ¹