(And the believers should not all) it is not permissible for all the believers to (go out to fight) and leave the Prophet (pbuh) alone in Medina. (Of every troop of them, a party only should go forth) while another party should stay in Medina, (that they (who are left behind) may gain sound knowledge in religion) that they may learn the religion from the Prophet (pbuh) (and that they may warn) and that they may inform (their folk when they return to them) from their military expedition, (so that they may beware) in order to know that which they have been commanded to do and that which they have been prohibited from doing. It is also said that this verse was revealed about the Banu Asad who came to the Prophet (pbuh) in Medina after being hit by drought. Their arrival caused prices in Medina to rise and all the streets became filthy with excrement. And so Allah prohibited them from this.