Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) sent out a detachment, and in the course of killing they went as far as the children. The Messenger of Allah (peace and blessings be upon him) said to them: "What happened with the children?" They said: "O Messenger of Allah! Are they not of the same stock as their fathers?" The Messenger of Allah (peace and blessings be upon him) said: "Are not the best of you from the very children of the polytheists? Indeed, every soul is born upon the fitrah (natural disposition) until its tongue begins to speak."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 828