Musannaf Ibn Abi Shayba 28:603

'Abd al-Rahim ibn Sulayman narrated to us, from Isma'il, from al-Hasan, from al-Aswad ibn Sari', who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "What is the matter with a people who have gone to extremes in killing, until they have killed children?" A man from the people said: "They are only the children of the polytheists." The Messenger of Allah (may Allah bless him and grant him peace) said: "Are your best ones not also children of the polytheists? There is no newborn who is born except upon the *fitra* (natural disposition), until he reaches the age where he expresses himself, or his parents make him a Jew or make him a Christian."