It is narrated from Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) that they went on a military expedition with the Messenger of Allah (peace and blessings be upon him) and met the enemy, and in the course of killing they went as far as the children. So they asked the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Every child is upon the fitrah (natural disposition) until his parents make him a Jew and a Christian."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 827