Sahih Muslim 6760
صحيح مسلم#6760SahihIt is reported on the authority of Abu Mu'awiya that (the Holy Prophet) said:Every new-born babe is born on the millat (of Islam and he) remains on this until his tongue is enabled to express himself. This hadith has been narratted on the authority of Abu Mu'awiya through another chain of transmitters (and the words are):" Every child is born but on this Fitra so long as he does not express himself with his tongue
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 46:38