Allah said,

(And there are among them Ummyyun people) meaning, among the People of the Book, as Mujahid stated. Ummyyun, is plural for Ummi, that is, a person who does not write, as Abu Al-`Aliyah, Ar-Rabi`, Qatadah, Ibrahim An-Nakha`i and others said. This meaning is clarified by Allah's statement,

(Who know not the Book) meaning, are they not aware of what is in it.

Ummi was one of the descriptions of the Prophet because he was unlettered. For instance, Allah said,

(Neither did you (O Muhammad) read any book before it (this Qur'an) nor did you write any book (whatsoever) with your right hand. In that case, indeed, the followers of falsehood might have doubted) (29:48).

Also, the Prophet said,

(We are an Ummi nation, neither writing nor calculating. The (lunar) month is like this, this and this (i.e. thirty or twenty-nine days.)

This Hadith stated that Muslims do not need to rely on books, or calculations to decide the timings of their acts of worship. Allah also said,

(He it is Who sent among the Ummiyyin ones a Messenger (Muhammad) from among themselves) (62:2).