Imam Malik's Muwatta, Chapter: Jihad

Yahya related to me from Malik that he had heard that Umar ibn Abd al-Aziz wrote to one of his governors, "It has been passed down to us that when the Messenger of Allah, may Allah bless him and grant him peace, sent out a raiding party, he would say to them, 'Make your raids in the name of Allah in the way of Allah. Fight whoever denies Allah. Do not steal from the booty, and do not act treacherously. Do not mutilate and do not kill children.' Say the same to your armies and raiding parties, Allah willing. Peace be upon you."