There is none born but is created to his true nature (Islam). It is his parents who make him a Jew or a Christian or a Magian quite as beasts produce their young with their limbs perfect. Do you see anything deficient in them? Then he quoted the Qur'an., The nature made by Allah in which He has created men there is no altering of Allah's creation; that is the right religion" (ar-Rum:

30)
حَدَّثَنَا حَاجِبُ بْنُ الْوَلِيدِ، حَدَّثَنَا مُحَمَّدُ بْنُ حَرْبٍ، عَنِ الزُّبَيْدِيِّ، عَنِ الزُّهْرِيِّ، أَخْبَرَنِي سَعِيدُ بْنُ الْمُسَيَّبِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّهُ كَانَ يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ مَا مِنْ مَوْلُودٍ إِلاَّ يُولَدُ عَلَى الْفِطْرَةِ فَأَبَوَاهُ يُهَوِّدَانِهِ وَيُنَصِّرَانِهِ وَيُمَجِّسَانِهِ كَمَا تُنْتَجُ الْبَهِيمَةُ بَهِيمَةً جَمْعَاءَ هَلْ تُحِسُّونَ فِيهَا مِنْ جَدْعَاءَ ‏"‏ ‏.‏ ثُمَّ يَقُولُ أَبُو هُرَيْرَةَ وَاقْرَءُوا إِنْ شِئْتُمْ ‏{‏ فِطْرَةَ اللَّهِ الَّتِي فَطَرَ النَّاسَ عَلَيْهَا لاَ تَبْدِيلَ لِخَلْقِ اللَّهِ‏}‏ الآيَةَ ‏.‏
Reference : Sahih Muslim 2658 b
In-book reference : Book 46, Hadith 34
USC-MSA web (English) reference : Book 33, Hadith 6423
  (deprecated numbering scheme)