Rav Saadia ben Yosef al-Fayyumi Gaon, the Rasag, was born in 882 (4642) in the village of Dilaz in the Fayyum district of Upper Egypt, hence his name "Fayyumi." According to later testimonies he was a descendant of the Tanna Rabbi Chanina ben Dosa, and he named one of his sons Dosa. As a young man he left Egypt for Eretz Yisrael, where he studied in Tiberias under Eli ben Yehuda HaNazir (Abu Kathir), and then traveled to Syria and Babylonia.

At the age of twenty he composed his first major work, the Hebrew dictionary Agron, and at twenty-three a polemic against the followers of Anan ben David, the beginning of his struggle against the Karaites in defense of the Oral Torah. Between 921 and 923, before he was Gaon, he led the sages of Babylonia against Aharon ben Meir, Gaon of Eretz Yisrael, in the controversy over the fixing of the calendar, writing the Sefer HaMoadim on the subject; his position prevailed and preserved the unity of the calendar.

On 1 Sivan 4688 (928), at the age of forty-six, the Exilarch David ben Zakkai appointed him Gaon of Sura, although he was a foreigner from Egypt, against the custom that reserved the office for the academy's own scholars. Under his leadership Sura again became the greatest Torah center of Babylonia. Two years later he refused to sign a ruling of the Exilarch that he considered unjust; the Exilarch deposed and excommunicated him, and each appointed a replacement for the other. They were reconciled in 937, and Saadia resumed his office.

A man of universal talents, he was in many fields the founder and the first: the first to study Hebrew grammar systematically, one of the first rabbis to write extensively in Judeo-Arabic, of whose literature he is considered the father, and the first to compose a systematic exposition of Jewish thought. Works: the Arabic translation of the Bible with commentary (the Tafsir), Sefer HaEmunot VehaDeot (The Book of Beliefs and Opinions, completed in 933), a siddur, works of halachah and grammar, piyyutim, and polemical writings against the Karaites and against Chivi al-Balkhi.

He passed away in Sura on the night of 26 Iyar 4702 (May 16, 942), of "black gall" according to Avraham ibn Daud, after fourteen years at the head of the academy and at less than sixty years of age, as his sons Sherit and Dosa wrote; Dosa later became Gaon of Sura himself. The traveler Benjamin of Tudela (12th century) saw his grave at Mata Mechasya (Sura) in Babylonia; later traditions place his grave at Mount Sinai or in the old cemetery of Safed.