It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: Every child is born upon the natural religion, that is, the religion of Islam. Later, his parents make him a Jew, a Christian, or a Magian. This is just like an animal giving birth to a complete offspring—do you see any among them with a cut ear? If you want confirmation of this, then recite this verse: {فِطْرَتَاللّٰہِالَّتِیْفَطَرَالنَّاسَعَلَیْھَالَاتَبْدِیْلَلِخَلْقِاللّٰہِ } (Adopt the nature of Allah, that is, this religion upon which He has created people; no one has the right to alter the creation of Allah.)
Hadith Referenceالفتح الربانی / كتاب النار والجنة / 13251