It is narrated from Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) that the Companions of the Prophet (peace and blessings be upon him) hastened in killing until they went as far as killing children. So the Messenger of Allah (peace and blessings be upon him) said: "Every child is born upon the fitrah (natural disposition) until his parents make him a Jew and a Christian."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 831