The continuing feud between the Aws and the Khazraj was probably the chief cause for several emissaries to invite Muhammad to Yathrib in order to adjudicate in disputed cases. Ibn Ishaq recorded that after his arrival in 622, Muhammad established a compact, the Constitution of Medina, which committed the Jewish and Muslim tribes to mutual cooperation. The nature of this document as recorded by Ibn Ishaq and transmitted by Ibn Hisham is the subject of dispute among modern historians, many of whom maintain that this "treaty" is possibly a collage of agreements, of different dates, and that it is not clear when they were made. Watt holds that the Qurayza and Nadir were probably mentioned in an earlier version of the Constitution requiring the parties not to support an enemy against each other.