From the seventh century C.E. on, canonical East-Syrian law developed in the shadow of Islamic rule as Islamic courts imposed new standards for dispute resolution. Yet, interactions between Muslim practices and East-Syrian law in the first centuries of Islam remain little studied. This article examines the evolution of East-Syrian canon law regarding oath-taking up to the early ninth century. It argues that formal oath prohibition, formulated by canon law based on the Gospel of Matthew, became controversial among jurists. At the end of the eighth and the beginning of the ninth century, Ishoʿ bar Nūn authorized this procedure, while Ishoʿbokht gave it theoretical foundations by assessing the historical significance of Jesus’s requirements and by appealing to the concept of necessity. The integration of oath-taking into the East-Syrian judicial process can therefore be interpreted as a response to its widespread use in Islamic courts, allowing Christians to better defend their cases.