Musannaf Ibn Abi Shayba 28:584

Abdullah ibn Numayr and Abu Usamah narrated to us, they said: Ubaydullah ibn Umar narrated to us, from Nafi’, from Ibn Umar, who said: A woman was found killed in one of the military expeditions of the Messenger of Allah, may Allah bless him and grant him peace. So the Messenger of Allah, may Allah bless him and grant him peace, forbade the killing of women and children.