(6) بَاب مَعْنَى كُلِّ مَوْلُودٍ يُولَدُ عَلَى الْفِطْرَةِ وَحُكْمِ مَوْتِ أَطْفَالِ الْكُفَّارِ وَأَطْفَالِ الْمُسْلِمِينَ
It 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."
Reference: Sahih Muslim 2658 f
In-book reference: Book 46, Hadith 38
USC-MSA web (English) reference(deprecated numbering scheme): Book 33, Hadith 6427