﴿ وَأَنزَلَ ٱلَّذِينَ ظَٰهَرُوهُم مِّنْ أَهْلِ ٱلْكِتَٰبِ مِن صَيَاصِيهِمْ وَقَذَفَ فِى قُلُوبِهِمُ ٱلرُّعْبَ فَرِيقًۭا تَقْتُلُونَ وَتَأْسِرُونَ فَرِيقًۭا ﴾

“and He brought down from their strongholds those of the followers of earlier reve­lation who had aided the aggressors, and cast terror into their hearts: some you slew, and some you made captive;”

And He brought down those of the People of the Scripture namely the Banū Qurayza who had supported them from their strongholds sayāsī is the plural of sīsa and is that in which one fortifies himself against an enemy and He cast terror into their hearts so that some of them you slew and these were the combatants and some of them you took captive namely their children.