Musannaf Ibn Abi Shayba 28:590

Yahya ibn Adam narrated to us, he said: Hasan ibn Salih narrated to us, from Khalid ibn al-Faraz, he said: Anas ibn Malik narrated to me, he said: I was the attendant of my companions, and when we would set out on a journey, we would encamp on the outskirts of Medina until the Messenger of Allah (may Allah bless him and grant him peace) would come out to us and say: "Set out in the name of Allah and in the path of Allah. You fight the enemies of Allah in the path of Allah. Do not kill an elderly person who is frail, nor a small child, nor a woman, and do not act treacherously (with the spoils of war)."