At a young age of 20 Saadia began composing his first great work, the Hebrew dictionary which he entitled "Agron". At 23 he composed a polemic against the followers of Anan ben David, particularly Solomon ben Yeruham, thus beginning the activity which was to prove important in opposition to Karaism, in defense of rabbinic Judaism. In the same year he left Egypt, and moved to the Land of Israel. Later, one of Saadia's chief disputants was the Karaite by the name of Abu al-Surri ben Zuṭa, who is referred to by Abraham ibn Ezra, in his commentary on the Pentateuch (Exo. 21:24, and Lev. 23:15 Ezra's second edition). In the year 928, at the age of thirty-six (variant: forty-six), David ben Zakkai, the Exilarch of Babylonian Jewry, petitioned Saadia to assume the honorary title of Gaon, where he was appointed that same year the Gaon of the Academy of Sura at Mata Mechasya, a position which he held for 14 years, until his death. After only two years of teaching, R. Saadia recused himself from teaching, because of a dispute that had fallen out between him and the Exilarch. During Saadia's absence, his post was occupied by R. Yosef, the son of R. Yaakov, the son of R. Natronai. At length, R. Saadia was reconciled with the Exilarch and returned to serve in his former position, although R. Yosef b. Yaakov also remained serving in his capacity as Gaon.