Musnad Ahmad 25:130

Abu Mu'awiyah narrated to us, Hajjaj narrated to us, from 'Ata', from Ibn 'Abbas, who said: "Najdah al-Haruri wrote to Ibn 'Abbas asking him about killing children, and about the khums (one-fifth share of spoils), to whom does it belong? And about the child, when does orphanhood cease for him? And about women, did he used to take them out, or were they present at the fighting? And about the slave, does he have a share in the spoils?" He said: So Ibn 'Abbas wrote to him: "As for the children: if you are Al-Khidr and can distinguish the disbeliever from the believer, then kill them. As for the khums: we used to say that it is for us, but our people claimed that it is not for us. As for the women, the Messenger of Allah (peace and blessings of Allah be upon him) used to take women out with him, so they would treat the sick and tend to the wounded, but they would not be present at the fighting. As for the child, orphanhood ceases for him when he reaches puberty. As for the slave, he has no share in the spoils, but they used to be given a small gift (radkh) from it."