The fiscal tradition regarding the taxation of the “Christians of the Banū Taġlib” is related to a ṣulḥ established by caliph ʿUmar b. al-Ḫaṭṭāb. He is said to have ordered to double the social contribution of Muslims, called ṣadaqa , in return for their renunciation of baptizing their children. This contribution analyzes the chronology of the emergence of this case in Abbasid literature. By studying the isnād on which scholars, beginning with the Grand Qāḍī Abū Yūsuf (d. 182/798), relied, I suggest a new theme at the end of the eighth century CE. On the one hand, all the issues of the ṣadaqa levying, the social bonds with Christian Arabs, and finally, the category of the Banū Taġlib itself, are related to the ongoing construction, and then to the freezing, of two social categories: the ethnical “arabness” and the genealogical tribal organization. On the other hand, Miaphysite ecclesiology confirms a consistent timeline for the rise of the Taglibōyē bishopric. In order to explain this late inrush of information, two events of ca. 153/770 and 171/787, respectively from the Syrian-Orthodox and the Arab-Muslim literatures, refer first to the migration/invasion of pastoralists and farmers of Banū Taġlib towards the north, and second to their anti- ṣadaqa revolt in the steppe of the hinterland of Mosul. The second occurrence takes place amidst numerous local insurgencies whose motivation are in part tribal, khariji, and – perhaps foremost – anti-fiscal. Indeed, the dynamics of formation of this peculiar Mosulian tribe were partly generated by the irruption of the state in Northern Iraq and the Jazīra during the 170s/790s, between al-Mahdī’s founding of Rāfiqa in ca. 154/772 and Hārūn al-Rashīd’s strengthening of administrative pressure. The key factor for state building at this time was the development of a new set of taxation on agricultural incomes of (Muslim) Arabs, called ṣadaqat al-māl or zakāt , whose first traces are attested in Middle Egypt during the late Marwānid period. A generation later, the anonymous author of Zuqnīn, who lived at the beginning of this period, is not only the first Syriac writer to mention the Taglibōyē, but also bears witness of the first extension of the levy of ṣadaqa to Northern Mesopotamia. He even gives data about its ex officio settlement ( taʿdīl ) as a non-proportional ( ʿalā misāḥa ) and in-cash tax, exactly the same as for the properties of the (Christian) Syrians. Both kinds of rural landlords probably petitioned against this system during the following decades, aiming to switch to a proportional ( muqāsama ) and in-kind method of taxation. This resistance perhaps involved the anti- ṣadaqa revolt of the Taġlib in 171/787, as it was decisive to transform the kharāj on the Muslims into a tenth ( ʿushr ). Whereas Abū Yūsuf suggests an analogical doubling of the tax on a Muslim land to convert it into kharāj for a Christian purchaser, the very idea of a double ṣadaqa as kharāj for Christian Arabs had perhaps been invented a very short time before.