Musnad Ahmad 25:1341

'Abd al-Rahman ibn Mahdi narrated to us, Jarir ibn Hazim narrated to us, from Qays ibn Sa'd, from Yazid ibn Hurmuz, who said: Najdah ibn 'Amir wrote to Ibn 'Abbas asking him about some things. I witnessed Ibn 'Abbas when he read his letter and when he wrote his response. He wrote to him: "You have asked me..." and he mentioned the hadith. He said: "And you asked whether the Messenger of Allah (may Allah bless him and grant him peace) used to kill any of the children of the polytheists. The Messenger of Allah (may Allah bless him and grant him peace) did not use to kill any of them. And you, do not kill any of them, unless you know about them what Al-Khidr knew about the boy when he killed him."