Aside from the general agreements, the chronicles by Ibn Ishaq and al-Waqidi contain a report that after his arrival, Muhammad signed a special treaty with the Qurayza chief Ka'b ibn Asad. Ibn Ishaq gives no sources, while al-Waqidi refers to Ka’b ibn Malik of Salima, a clan hostile to the Jews, and Mummad ibn Ka’b, the son of a Qurayza boy who was sold into slavery in the aftermath of the siege and subsequently became a Muslim. The sources are suspect of being against the Qurayza and therefore the historicity of this agreement between Muhammad and the Banu Qurayza is open to grave doubt. Among modern historians, R. B. Serjeant supports the historicity of this document and suggests that the Jews knew "of the penalty for breaking faith with Muhammad". On the other hand, Norman Stillman argues that the Muslim historians had invented this agreement in order to justify the subsequent treatment of the Qurayza. Watt also rejects the existence of such a special agreement but notes that the Jews were bound by the aforementioned general agreement and by their alliance to the two Arab tribes not to support an enemy against Muhammad. Serjeant agrees with this and opines that the Qurayza were aware of the two parts of a pact made between Muhammad and the Jewish tribes in the confederation according to which "Jews having their religion and the Muslims having their religion excepting anyone who acts wrongfully and commits crime/acts treacherously/breaks an agreement, for he but slays himself and the people of his house."