Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Every child of the children of Adam is born upon the fitrah (natural disposition) until his parents make him a Jew, a Christian or a Magian, just as an animal gives birth to a sound, whole young one. Do you find any with its ear cut off?" Then Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: If you wish, recite (this verse): "(Adhere to) the fitrah of Allah upon which He created mankind. There is no altering the creation of Allah. That is the upright religion, but most people do not know" [Surah al-Rum: 30].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12138