A group of people often mentioned together with the Jews and Christians. The Qur'an says that the righteous among them will be rewarded. However, no trace of them remains and even their history is little known. Tisdall quotes Muslim authorities:

The SABAEANS have disappeared. No trace of them anywhere remains, and even of their history but little is known. We are told by Eastern authorities 1. that they were the first of all peoples who inhabited Syria; that they derived their faith from Seth and Idris; and that they possessed a book called Pages of Seth, In which were inculcated righteousness, truth, bravery, care of the poor, and avoidance of evil. They had seven times for prayer, five of which were at the same hour as chosen by the Prophet. They prayed also for the dead, but without prostration; fasted thirty days from night to sunrise, and also if any new moon rose badly, for the remaining day of the month; observed Fed from the setting of their five stars; and venerated the Kaaba. Hence we see that the Sabaeans kept many observances still in force among the Moslems.

[footnote 1: What follows is from Abu Feda, who quotes from Abu Isa al-Maghrabi]
(W. St. Clair-Tisdall, Sources of Islam)