In the early ninth century, the East Syrian patriarch Timothy I promulgated a communal law prohibiting marriages between cousins. A deacon of ?ira questioned this prohibition (which likely conflicted with social practice in many East Syrian communities) in a letter to Timothy’s successor Isho‘barnun, who was quick to rescind the stricture. Isho‘barnun’s concerns were not the exigencies of local custom, however; from his point of view, Timothy’s new law was irreconcilable with scripture and East Syrian exegetical tradition. Timothy had arrived at his ban on cousin marriage by a method of legal reasoning, basing it analogically on other kinship prohibitions. Isho‘barnun, however, knew an East Syrian exegetical tradition that rendered Abraham and Sarah cousins, rather than uncle and niece as in Jewish exegesis, and thereby absolved the biblical patriarchs of what he saw as an unlawful uncle-niece marriage. For Isho‘barnun, Timothy’s attempt to make East Syrian family law more systematically consistent by banning cousin marriage did no more than implicate the biblical patriarchs in a second incestuous marriage after exegetes had gone to lengths to explain them out of a first. This dispute over cousin marriage in medieval East Syrian legal texts represents a jockeying effort to establish the authoritative sources from which East Syrian law should be drawn, and parallels contemporary developments in Islamic law. Where Timothy based his prohibition of cousin marriage on personal ecclesiastical authority and a method of analogical reasoning, Isho‘barnun maintained that any new laws on Christian marital practice had to be in line with God’s allowances in scripture and exegetical authorities’ understandings thereof. Isho‘barnun and Timothy’s dispute thus brought to the fore different conceptions of the degrees to which received tradition and specialists’ reasoning should shape communal regulations and inform the practices of the faithful. When considered alongside contemporary disputes between ahl al-ra’y and ahl al-?adith among the early Muslim jurisprudents, the East Syrians’ conflict is indicative of a broader shift on the part of communal elites in ‘Abbasid Iraq toward conceptions of religious communities as law-bounded.