It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that whenever the Messenger of Allah (peace and blessings be upon him) dispatched his armies, he would say: "Go forth in the name of Allah, and fight in the path of Allah those who disbelieve in Allah. Do not transgress, do not mutilate (the bodies of the dead), do not kill children, and do not kill the people of the hermitages."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11513