The Siddur of Rav Amram Gaon סדר רב עמרם גאון

The community of Barcelona asked the head of Sura to send them the order of prayers for the whole year. Rav Amram answered not with a list of texts but with a book: the prayers together with the laws about them—when they are said, what may be omitted, what the congregation answers.

Before him prayer books as books did not exist: the prayers were known by heart and handed on orally, and variants multiplied. Rav Amram's answer spread through the communities and became the basis of almost every later order of prayer, Ashkenazi and Sephardi alike. A century later Rav Saadya Gaon would compile his own siddur, but this one was first.