The Messenger of Allah ﷺ said, "Every child is born upon the natural disposition, then his parents make him a Jew, a Christian, or a Zoroastrian. It is like an animal that gives birth to another animal. Do you find any of them mutilated at birth?"
حدثنا حسين حدثنا ابن أبي ذئب عن الزهري عن أبي سلمة
قال رسول الله ﷺ كل مولود يولد على الفطرة فأبواه يهودانه وينصرانه ويمجسانه كمثل البهيمة تنتج البهيمة هل تكون فيها جدعاء
Its isnad is sahih. It has already appeared at 7181. Bukhari related it in 2/125, in the Book of Funerals, in the chapter on what is said about the children of polytheists, and Muslim related it in 4/2048, number 2658, in the Book of Divine Decree, on the meaning of, "Every child is born upon the fitrah." Tirmidhi related similarly in 4/447, number 2138, and Abu Dawud in 4/229, number 4712, in the Book of Sunnah, in the chapter on the offspring of polytheists.The meaning is that when an animal gives birth, it does not give birth to an animal with a cut ear, rather it is its owner who cuts its ear.