Ikrimah (may Allah have mercy on him) narrates 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 an army (in the narration of Ibn Abi Uways: whenever the Prophet (peace and blessings be upon him) dispatched his armies, he would say): "Go out in the name of Allah, and fight those who disbelieve in Allah. Do not commit treachery, mutilation or theft from the spoils, and do not kill children or monks."
However, in the narration of al-Misri these words, "And do not steal from the spoils," are not present.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18154