Sahih Muslim 2658 e
Abu Huraira reported Allah's Messenger (ﷺ) as saying:
No babe is born but upon Fitra. It is his parents who make him a Jew or a Christian or a Polytheist. A person said: Allah's Messenger, what is your opinion if they were to die before that (before reaching the age of adolescence when they can distinguish between right and wrong)? He said: It is Allah alone Who knows what they would be doing.
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ،
 قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ مَا مِنْ مَوْلُودٍ إِلاَّ يُلِدَ عَلَى الْفِطْرَةِ فَأَبَوَاهُ يُهَوِّدَانِهِ
 وَيُنَصِّرَانِهِ وَيُشَرِّكَانِهِ ‏"‏ ‏.‏ فَقَالَ رَجُلٌ يَا رَسُولَ اللَّهِ أَرَأَيْتَ لَوْ مَاتَ قَبْلَ ذَلِكَ قَالَ ‏"‏ اللَّهُ
 أَعْلَمُ بِمَا كَانُوا عَامِلِينَ ‏"‏ ‏.‏