However, after the 25 days siege, the situation of the Banu Qurayza was now hopeless and the next morning the they officially surrendered.
In the morning they submitted to the apostle's judgement and al-Aus leapt up and said, 'O Apostle, they are our allies, not allies of Khazraj, and you know how you recently treated the allies of our brethren.' Now the apostle had besieged B. Qaynuqa` who were allies of al-Khazraj and when they submitted to his judgement `Abdullah b. Ubayy b. Salul had asked him for them and he gave them to him; so when al-Aus spoke thus the apostle said: 'Will you be satisfied, O Aus, if one of your own number pronounces judgement on them ?' When they agreed he said that Sa`d b. Mu`adh was the man. [Sirat, p. 463]
Let us summarize the observations so far.
1. Remember the phrase from the beginning? It said "and when they felt sure that the apostle would not leave them until he had made an end of them". This indicates the B. Qurayza knew what Muhammad was up to.
2. When Abu Lubaba a Muslim who fought on Muhammad's side was asked by them what will happen and what they should do, he indicates slaughter. This is not a prejudiced fear of the enemy (we might often exaggerate what the enemies might do to us and have an image of them worse than their actual nature), this now is the expectation or knowledge of a Muslim about his own prophet.
3. When they surrender to Muhammad, the immediate reaction of the al-Aus is to intercede for them. Why would they remind the Prophet of what he had done earlier? Does that look like they expected him to be naturally merciful? If they expected him to be merciful why did they not rest in assurance that Muhammad would act with more mercy than any of them would have? This kind of immediate reaction shows they feared for their friends now that they have surrendered, and they bring the best argument they have to sway Muhammad from his plan. They appeal to his justice regarding how he treats his friends and since he did hear the intercession of the other tribe earlier he shouldn't be seen playing favorites with them and also acknowledge their earlier alliance with the Banu Qurayza. They do not appeal to his nature of mercy towards the Jews, but to his nature as statesman who needs to be impartial with all his companions. This reaction certainly shows that the al-Aus feared for their friends and did not have the impression Muhammad had mercy on his mind.
The people from al-Aus reminded Muhammad of his earlier decision in regard to the other Jewish tribe of the Banu Qaynuqa`. In order to understand this reference, let me quote what this is all about.
`Asim b. `Umar b. Qatada said that the B. Qaynuqa` were the first of the Jews to break their agreement with the apostle and to go to war, between Badr and Uhud, and the apostle besieged them until they surrendered unconditionally. `Abdullah b. Ubayy b. Salul went to him when God had put them in his power and said, 'O Muhammad, deal kindly with my clients' (now they were allies of Khazraj), but the apostle put him off. He repeated the words, and the apostle turned away from him, whereupon he thrust his hand into the collar of the apostle's robe; the apostle was so angry that his face became almost black. He said, 'Confound you, let me go.' He answered, 'No, by God, I will not let you go until you deal kindly with my clients. Four hundred men without mail and three hundred mailed protected me from all mine enemies; would you cut them down in one morning? By God, I am a man who fears that circumstances may change.' The apostle said, 'You can have them.' [Sirat, p. 363]
What impression do we get of Muhammad's intention? How easily was he convinced to be merciful to them?
When `Abdullah interceded for them and insisted on it, Muhammad became extremely angry and it took great courage on the part of `Abdullah to stick to his request and even use physical force against Muhammad to hold him back from massacering all of the tribe. This confirms that Muhammad had the intention of killing the Banu Qaynuqa` from the very beginning and only was hindered by others from doing so. Muhammad was not easily swayed in his intention, but it took considerable effort hindering him.
There was another tribe of Jews in Medina. the Banu al-Nadir. They are not refered to in our story, but they also contribute something to our understanding.
Concerning B. al-Nadir the Sura of Exile came down in which is recorded how God wreaked His vengeance on them and gave His apostle power over them and how He dealt with them. God said: 'He it is who turned out those who disbelieved of the scripture people from their homes to the first exile. ... 'So consider this, you who have understanding. Had not God prescribed deportation against them,' which was vengeance from God, 'He would have punished them in this world,' i.e. with the sword, 'and in the next world there would be the punishment of hell' as well. [Sirat, p. 438]
It seems not clear what but "something" happened to make Muhammad change his mind and which then is justified with this sura. (Sorry, but it was not God, not even in this good thing of sparing their life. God doesn't give revelations to people who are out to murder). But even in this event, the Sirat testifies that Muhammad originally intended to kill them all.
Therefore, the historical records regarding the two "spared" tribes actually confirm that Muhammad was planning to kill all of the Qurayza just as he intended to deal with the tribes of the Banu Qaynuqa` and Banu al-Nadir. For some reason he was prevented in the first two cases. Forcibly in respect to the Banu Qainuqa, and we don't really know why in regard to the Banu al-Nadir. But: Deeds are judged by intention.
The impression becomes strong that with the third tribe Muhammad now wants to make sure that he won't be losing out on his plans again. To me, the way he asks the al-Aus and then chooses Sa`d b. Mu`adh afterwards, seems planned to prevent another tribe from escaping, and his intentions be thwarted again.
Let us have a second look at the paragraph already quoted in the above.
In the morning they submitted to the apostle's judgement and al-Aus leapt up and said, 'O Apostle, they are our allies, not allies of Khazraj, and you know how you recently treated the allies of our brethren.' Now the apostle had besieged B. Qaynuqa` who were allies of al-Khazraj and when they submitted to his judgement `Abdullah b. Ubayy b. Salul had asked him for them and he gave them to him; so when al-Aus spoke thus the apostle said: 'Will you be satisfied, O Aus, if one of your own number pronounces judgement on them ?' When they agreed he said that Sa`d b. Mu`adh was the man. [Sirat, p. 463]
The next crucial player in the tragedy enters the scene. Who is Sa`d b. Mu`adh? Why is he chosen by Muhammad? Since there is a wealth of material available in the Sirat and Hadith about this man we can answer this question with high confidence.
Part 2: Who is Sa`d bin Mu`adh?
https://www.answering-islam.org/Muhammad/Jews/BQurayza/banu1.html