{ It is not for all of the Believers to mobilize; if a small band from among each group were not to mobilize, in order to gain knowledge in religion, and make their people aware, when they return to them, that perchance they might take caution } Al-tawba 122

The meaning is that it is not permissible for the Believers to mobilize in their entirety for attacks and jihad, but rather they are to form two groups. One group stays to serve the Messenger, and the other group mobilizes to attack. This is because at that time Islam was in need of attacking and jihad, and conquering the Disbelievers, but ordinances were also coming to pass, and divinely prescribed laws were being sent down, and the Muslims were in need of people to remain present with the Messenger (peace be upon him) and learn these laws and guard these ordinances, and then to communicate them to those who were absent.