I had always taken for granted that the men of Banu Qurayza were killed, following that tribes violation of its military alliance with the Muslims and siding with the Quraysh when Medina was attacked.
However, the following article has made me rethink this - apparently, the accounts of the history of Muslim society are not subject to the same stringest standards of testing authenticity as Hadith are. More to the point, Imam Malik is claimed to have accused the primary narrator of the story of the Banu Qurayza, Ibn Ishaq, as being an outright liar who would base some of his accounts on what he heard from Jews.
http://www.haqq.com.au/~salam/misc/qurayza.html
IT IS WELL KNOWN THAT at the advent of Islam there were three Jewish tribes who lived in Yathrib (later Medina), as well as other Jewish settlements further to the north, the most important of which were Khaybar and Fadak. It is also generally accepted that at first the Prophet Muhammad hoped that the Jews of Yathrib, as followers of a divine religion, would show understanding of the new monotheistic religion, Islam. However, as soon as these tribes realized that Islam was being firmly established and gaining power, they adopted an actively hostile attitude, and the final result of the struggle was the disappearance of these Jewish communities from Arabia proper.
The biographers of the Prophet, followed by later historians, tell us that Banu Qaynuqa.,1 and later Banu al-Nadir,2 provoked the Muslims, were besieged, and in turn agreed to surrender and were allowed to depart, taking with them all their transportable possessions. Later on Khaybar3 and Fadak4 were evacuated. According to Ibn Ishaq in the Sira,5 the third of the Jewish tribes, Banu Qurayza, sided with the Qura****es and their allies, who made an unsuccessful attack on Medina in an attempt to destroy Islam. This, the most serious challenge to Islam, failed, and the Banu Qurayza were in turn besieged by the Prophet. Like Banu al-Nadir, in time they surrendered, but unlike the Banu al-Nadir, they were subjected to the arbitration of Sa’d b. Mu’adh, a member of the Aws tribe, allies of Qurayza. He ruled that the grown-up males should be put to death and the women and children subjected to slavery. Consequentiy, trenches were dug in the market-place in Medina, and the men of Qurayza were brought out in groups and their necks were struck.6 Estimates of those killed vary from 400 to 900.
On examination, details of the story can he challenged. It can be demonstrated that the assertion that 600 or 800 or 9007 men of Banu Qurayza were put to death in cold blood can not be true; that it is a later invention; and that it has its source in Jewish traditions. Indeed the source of the details in earlier Jewish history can be pointed out with surprising accuracy.
The Arabic sources will now be surveyed, and the contribution of their Jewish informants will be discussed. The credibility of the details will then be assessed, and the prototype in earlier Jewish history pin-pointed.
The earliest work that we have, with the widest range of details, is Ibn Ishaq’s Sira, his biography of the Prophet. It is also the longest and the most widely quoted. Later historians draw, and in most cases depend on him.8 But Ibn Ishaq died in 151 A.H., i.e. 145 years after the event in question. Later historians simply take his version of the story, omitting more or less of the detail, and overlooking his uncertain list of authorities. They generally abbreviate the story, which appears just as one more event to report. In most cases their interest seems to end there. Some of them indicate that they are not really convinced, but they are not prepared to take further trouble. One authority, Ibn Hajar, however, denounces this story and the other related ones as “odd tales”.9 A contemporary of Ibn Ishaq, Malik,10 the jurist, denounces Ibn Ishaq outright as "a liar"11 and "an impostor"12 just for transmitting such stories.
It must be remembered that historians and authors of the Prophet’s biography did not apply the strict rules of the “traditionists”. They did not always provide a chain of authorities, each of whom had to be verified as trustworthy and as certain or likely to have transmitted his report directly from his informant, and so on. The attitude towards biographical details and towards the early events of Islam was far less meticulous than their attitude to the Prophet’s traditions, or indeed to any material relevant to jurisprudence. Indeed Ibn Ishaq’s account of the siege of Medina and the fall of the Banu Qurayza is pieced together by him from information given by a variety of persons he names, including Muslim descendants of the Jews of Qurayza.
Against these late and uncertain sources must be placed the only contemporary and entirely authentic source, the Qur’an. There, the reference in Sura XXXIII, 26 is very brief:
“He caused those of the People of the Book who helped them (i.e. the Quraysh) to come out of their forts. Some you killed, some you took prisoner.” There is no reference to numbers.
Ibn Ishaq sets out his direct sources as he opens the relevant chapter on the siege of Medina. These were: a client of the family of al-Zubayr and others whom he “did not suspect”. They told parts of the story on the authority of 'Abdullah b. Ka’b b. Malik, al Zuhri, 'Asim b. 'Umar b. Qatada, 'Abdullab b. Abi Bakr, Muhammad b. Ka’b of Qurayza, and “others among our men of learning”, as he put it. Each of these contributed to the story, so that Ibn Ishaq’s version is the sum total of the collective reports, pieced together. At a later stage Ibn Ishaq quotes another descendant of Qurayza, 'Attiyya13 by name, who had been spared, and, directly, a certain descendant of al-Zabir b. Bata, a prominent member of the tribe of Qurayza who figures in the narrative.
The story opens with a description of the effort of named Jewish leaders to organize against the Muslims an alliance of the hostile forces. The leaders named included three from the Banu al-Nadir and two of the tribe of Wa’il, another Jewish tribe; together with other Jewish fellow-tribesmen unnamed. Having persuaded the neighbouring Bedouin tribes of Ghatafan, Murra, Fazara, Sulaym, and Ashja’ to take up arms, they now proceeded to Mecca where they succeeded in persuading the Quraysh. Having gathered together a besieging force, one of the Nadir leaders, Huyayy b. Akhtab, in effect forced himself on the third Jewish tribe still in Medina, the Banu Qurayza, and, against the better judgement of their leader, Ka’b b. Asad, he persuaded them to break faith with the Prophet in the hope, presented as a certainty, that the Muslims would not stand up to the combined attacking forces and that Qurayza and the other Jews would be restored to independent supremacy. The siege of Medina failed and the Jewish tribes suffered for their part in the whole operation.
The attitude of scholars and historians to Ibn lshaq’s version of the story has been either one of complacency, sometimes mingled with uncertainty, or at least in two important cases, one of condemnatlon and outright rejection.
The complacent attitude is one of accepting the biography of the Prophet and the stories of the campaigns at they were received by later generations without the meticulous care or the application of the critical criteria which collectors of traditions or jurists employed. It was not necessary to check the veracity of authorities when transmitting or recording parts of the story of the Prophet’s life.14 It was not essential to provide a continuous chain of authorities or even to give authorities at all. That is obvious in Ibn Ishaq’s Sira. On the other hand reliable authority and a continuous line of transmission were essential when law was the issue. That is why Malik the jurist had no regard for Ibn Ishaq.15
One finds, therefore, that later historians and even exegetes either repeat the very words of Ibn Ishaq or else abbreviate the whole story. Historians gave it, as it were, a cold reception. Even Tabari, nearly 150 years after Ibn Ishaq, does not try to find other versions of the story as he usually does. He casts doubt by his use of the words, “Waqidi alleged (za’ama) that the Prophet caused trenches to be dug.” Ibn ai-Qayyim in Zad al-ma’ad makes only the briefest reference and he ignores altogether the crucial question of numbers. Ibn Kathir even seems to have general doubt in his mind because he takes the trouble to point out that the story was told on such “good authority” as that of 'A’isha.16
Apart from mild complacency or doubtful acceptance of the story itself, Ibn Ishaq as an author was in fact subjected to devastating attacks by scholars, contemporary or later, on two particular accounts. One was his uncritical inclusion in his Sira of so much spurious or forged poetry;17 the other his unquestioning acceptance of just such a story as that of the slaughter of Banu Qurayza.