It is narrated from Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) sent out a detachment, and at their hands even the children of the polytheists were killed. This reached the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "What is the matter with people that they have killed even the children of the polytheists?" They said: "O Messenger of Allah", and after that they said something else. He (peace and blessings be upon him) said: "Are not the best of you from the very children of the polytheists? By the One in whose hand is my soul, every child is born upon the fitrah (natural disposition) until his parents make him a Jew or a Christian."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 822