Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) said: I went on a military expedition with the Messenger of Allah (peace and blessings be upon him), and Allah granted them victory. Then some people went as far as killing children. 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 their killing has gone as far as killing children?" A man said: "O Messenger of Allah! They are only the children of the polytheists." He (peace and blessings be upon him) said: "The best of you are from the very children of the polytheists. Listen! Children are not to be killed. Every soul is born upon the fitrah (natural disposition) until its tongue begins to speak; then its parents make it a Jew and a Christian."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 825