Musnad Ahmad 30:677

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, from Umar ibn Habib, from Amr ibn Dinar, from Tawus, from Abu Hurayrah, that the Prophet, may God bless him and grant him peace, said: "Every newborn is born upon the natural disposition (fitrah), then his parents make him a Jew or make him a Christian. Like livestock, they give birth to offspring that are intact, then you slit their ears."