Sahih Muslim , Book 33, Hadith 6430 (USC-MSA) ⓘ
Abu Huraira reported that Allah's Messenger was asked about the children of the polytheists, whereupon he said: It is Allah Who knows best what they would be doing.
حَدَّثَنَا أَبُو الطَّاهِرِ، أَخْبَرَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي ابْنُ أَبِي ذِئْبٍ، وَيُونُسُ، عَنِ ابْنِ، شِهَابٍ عَنْ عَطَاءِ بْنِ يَزِيدَ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم سُئِلَ عَنْ أَوْلاَدِ الْمُشْرِكِينَ فَقَالَ " اللَّهُ أَعْلَمُ بِمَا كَانُوا عَامِلِينَ " .
also referenced under: Book 46, Hadith 41 (In-Book) & Hadith 2659 (Reference)