Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) said that on the day of Hunayn the Messenger of Allah (peace and blessings be upon him) sent out a small detachment, who fought the polytheists, and in the fighting killed their children too. When they came back, he (peace and blessings be upon him) asked: "What led you to kill the children?" They said: O Messenger of Allah, they were the children of the polytheists. He (peace and blessings be upon him) said: "Are your best people anything but the children of polytheists? By the One in whose hand is the soul of Muhammad (peace and blessings be upon him), every soul is born upon the fitrah (natural disposition) until its tongue expresses itself clearly."
Imam al-Shafi'i (may Allah have mercy on him) said, in the narration of Abu Abd al-Rahman: It is the fitrah upon which Allah, the Lord of Might, created people. No ruling applies to them as long as they have not spoken clearly; they are counted as following their parents.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18334