Sahih Muslim 2658 f
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."
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا أَبُو مُعَاوِيَةَ، ح وَحَدَّثَنَا ابْنُ،
 نُمَيْرٍ حَدَّثَنَا أَبِي كِلاَهُمَا، عَنِ الأَعْمَشِ، بِهَذَا الإِسْنَادِ ‏.‏ فِي حَدِيثِ ابْنِ نُمَيْرٍ ‏"‏ مَا مِنْ مَوْلُودٍ
 يُولَدُ إِلاَّ وَهُوَ عَلَى الْمِلَّةِ ‏"‏ ‏.‏ وَفِي رِوَايَةِ أَبِي بَكْرٍ عَنْ أَبِي مُعَاوِيَةَ ‏"‏ إِلاَّ عَلَى هَذِهِ الْمِلَّةِ
 حَتَّى يُبَيِّنَ عَنْهُ لِسَانُهُ ‏"‏ ‏.‏ وَفِي رِوَايَةِ أَبِي كُرَيْبٍ عَنْ أَبِي مُعَاوِيَةَ ‏"‏ لَيْسَ مِنْ مَوْلُودٍ يُولَدُ
 إِلاَّ عَلَى هَذِهِ الْفِطْرَةِ حَتَّى يُعَبِّرَ عَنْهُ لِسَانُهُ ‏"‏ ‏.‏