Hasdai was very active on behalf of his co-religionists and Jewish science. Allegedly, when he heard that in Central Asia there was a Jewish state with a Jewish ruler, he desired to enter into correspondence with this monarch; and when the report of the existence of the Khazar state was confirmed by two Jews, Mar Saul and Mar Joseph, who had come in the retinue of an embassy from the Croatian king to Córdoba, Hasdai entrusted to them a letter, written in good Hebrew addressed to the Jewish king, in which he gave an account of his position in the Western state, described the geographical situation of Andalusia and its relation to foreign countries, and asked for detailed information in regard to the Khazars, their origin, their political and military organization, etc. ("See also the Khazar Correspondence.") Historian Shaul Stampfer has questioned the authenticity of letter said to have been received from the Khazar King, citing numerous linguistic and geographic oddities amid a flourishing of pseudo-historiographic texts and forgeries in medieval Spain.