Musnad Ahmad 1142:2

'Abd al-Razzaq narrated to us, from Ma'mar, who said: Al-Zuhri said: Ibn Ka'b ibn Malik informed me, from his paternal uncle: "That the Prophet, peace and blessings of Allah be upon him, when he sent [an expedition] to Ibn Abi al-Huqayq in Khaybar, he forbade the killing of women and children."