Musnad Abu Ya'la al-Mawsili 110:223

Zuhayr narrated to us, Isma'il bin Abi Uways narrated to us, he said: Ibrahim bin Isma'il narrated to me, from Dawud bin al-Husayn, from 'Ikrimah, from Ibn 'Abbas, from the Messenger of Allah (may Allah bless him and grant him peace): That when he would send out his armies, he would say: "Go forth in the name of Allah, and fight in the way of Allah against those who disbelieve in Allah. Do not act treacherously, do not mutilate, do not steal from the spoils (ghulul), and do not kill children nor the inhabitants of the hermitages."