Musannaf Ibn Abi Shayba 36:364

'Abd al-Rahim narrated to us, from Ibn Ishaq, from al-Zuhri, from 'Abd Allah ibn Ka'b ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) forbade the group who were sent to Ibn Abi al-Huqayq in Khaybar to kill him; so he forbade them from killing women and children.