From Sahih Bukhari (Hadith 4775):
Narrated Abu Huraira:Allah's Messenger (ﷺ) said, "No child is born except on Al-Fitra (Islam) and then his parents make him Jewish, Christian or Magian, as an animal produces a perfect young animal: do you see any part of its body amputated?" Then he recited, 'The religion of pure Islamic Faith (Hanifa),(i.e. to worship none but Allah), The pure Allah's Islamic nature with which He (Allah) has created mankind. Let There be no change in Allah's religion (i.e. to join none in Allah's worship). That is the straight religion; but most of men know not
Reference: Bukhari 4775