Musnad Ahmad 25:1405

Sufyan narrated to us, Ismail bin Umayyah narrated to us, from Sa'id bin Abi Sa'id al-Maqburi, from Yazid bin Hurmuz, who said: Najdah wrote to Ibn Abbas asking him about the killing of children. He wrote back to him: You wrote asking me about the killing of children, "and the Messenger of Allah (peace be upon him) did not use to kill them." And as for you, do not kill them, unless you know about them what the companion of Musa knew about the boy.