Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) narrates: The Messenger of Allah (peace and blessings be upon him) sent out a detachment. They met the enemy, and in the course of killing they went as far as the children. When the detachment returned, this reached the Prophet (peace and blessings be upon him), and he (peace and blessings be upon him) said: "Did I not forbid you?" They said: "They were only the children of the polytheists." He (peace and blessings be upon him) said: "Are not the best of you from the very children of the polytheists?" Then he (peace and blessings be upon him) ordered a caller, and he called out: "Listen! Every child is born upon the fitrah (natural disposition)."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 830