Sulayman bin Buraydah (may Allah have mercy on him) narrates from his father (Sayyiduna Buraydah, may Allah be pleased with him) that whenever the Messenger of Allah (peace and blessings be upon him) appointed someone as commander of an army or a detachment, he would advise him to fear Allah and to treat the Muslims well. Then he would say: "Go on campaign in the name of Allah, strive in the path of Allah, and fight those who disbelieve in Allah. Go on campaign! Do not misappropriate (the spoils), do not betray, do not mutilate, and do not kill a child."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18046