Musnad Ahmad 25:869

Abu al-Qasim ibn Abi al-Zinad narrated to us, he said: Ibn Abi Habibah informed me, from Dawud ibn al-Husayn, from Ikrimah, from Ibn Abbas, he said: When the Messenger of Allah, peace and blessings of Allah be upon him, would send out his armies, he would say: "Go forth in the name of Allah, fighting in the way of Allah against those who disbelieve in Allah. Do not act treacherously, do not exceed the limits, do not mutilate, do not kill children, and do not kill the inhabitants of the hermitages."