It was narrated that Ibn 'Abbas (رضي الله عنه) said: When the Messenger of Allah (ﷺ) sent out his army, he would say:

"Go out in the Name of Allah, fighting for the sake of Allah those who disbelieve in Allah. Do not be treacherous, do not steal from the war booty, do not mutilate (the enemy slain), and do not kill children or those who dwell in hermitages (i.e., monks)."