Sahih Muslim 2661
Sahih Muslim · Book 46: The Book of Destiny · The Meaning Of "Every Child Is Born In A State Of Fitrah" And The Ruling On The Dead Children Of The Disbelievers And Of The Muslims.
Ibn Abbas reported that Allah's Messenger (ﷺ) was asked about the children of the polytheists, whereupon he said:
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى ، أَخْبَرَنَا أَبُو عَوَانَةَ ، عَنْ أَبِي بِشْرٍ ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ ، عَنِ ابْنِ عَبَّاسٍ ، قَالَ سُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ أَطْفَالِ الْمُشْرِكِينَ قَالَ " اللَّهُ أَعْلَمُ بِمَا كَانُوا عَامِلِينَ إِذْ خَلَقَهُمْ " .
It is Allah alone Who knows what they would be doing according to their creation.