Musnad Abu Ya'la al-Mawsili 159:58

Wahb ibn Baqiyyah narrated to us, Khalid informed us, from Abd al-Rahman, from al-A'raj, from Abu Hurayrah, from the Prophet (peace be upon him), who said: "Every newborn is born upon the *fitrah* (natural disposition), then his parents make him a Jew or a Christian, just as animals produce offspring that are whole—do you perceive any that are mutilated?" They said, "What about those who die [in childhood], O Messenger of Allah?" He said, "Allah knows best what they would have been doing."