It has long been considered that Arabic algebra scarcely left any traces in mathematical literature of Hebrew expression. Thanks to the unpublished sources we have discovered, and to an attentive examination of already-known texts, one can no longer subscribe to such a judgement. The evidence we examine in this first article sheds light on the circulation, in erudite Jewish circles, of Arabic algebraic knowledge in Spain, Italy, Provence, and Sicily, between the 12th and the 14th centuries. The Epistle on number by the Castillian astronomer Isaac ben Salomon al-Aḥdab was written in Sicily at the end of the 14th century, and based on the Talkhīṣ a'māl al-ḥisāb of Ibn al-Bannā' (1256-1321). That part of the Epistle that is devoted to algebra follows the tradition of al-Karajī. It offers, for the first time in Hebrew, a rational presentation of arithmetical operations extended to algebraic expressions.