Every soul is born upon the fitrah

It is narrated from Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) sent a military detachment on the day of Hunayn (or of some battle). They fought the polytheists until the killing reached even the children (offspring). When they returned, the Prophet (peace and blessings be upon him) asked: "What drove you to kill the children?" They said: "O Messenger of Allah, they were only the children of the polytheists." He (peace and blessings be upon him) said: "Are not the best of you the children of the polytheists? By the One in whose hand is the soul of Muhammad, every child that is born is born upon the natural disposition (fitrah) of Islam, until its tongue (becoming able to speak) expresses it."

Something similar is narrated from Sayyiduna al-Aswad bin Sari' (may Allah be pleased with him).
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.