Muwatta Imam Malik Book of Burials, Hadith 53

Yahya related to me from Malik from Abu'z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah ﷺ said, "Every child is born on the fitra and it is his parents who make him a jew or a christian. Just as a camel is born whole - do you perceive any defect?" They said, "Messenger of Allah, what happens to people who die when they are (very) young?" He said, "Allah knows best what they used to do."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنِ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ : " كُلُّ مَوْلُودٍ يُولَدُ عَلَى الْفِطْرَةِ فَأَبَوَاهُ يُهَوِّدَانِهِ أَوْ يُنَصِّرَانِهِ، كَمَا تُنَاتَجُ الإِبِلُ مِنْ بَهِيمَةٍ جَمْعَاءَ هَلْ تُحِسُّ فِيهَا مِنْ جَدْعَاءَ " . قَالُوا : يَا رَسُولَ اللَّهِ أَرَأَيْتَ الَّذِي يَمُوتُ وَهُوَ صَغِيرٌ قَالَ : " اللَّهُ أَعْلَمُ بِمَا كَانُوا عَامِلِينَ " .