Sahih Muslim , Book 33, Hadith 6433 (USC-MSA) ⓘ
Ibn Abbas reported that Allah's Messenger was asked about the children of the polytheists, whereupon he said: It is Allah alone Who knows what they would be doing according to their creation.
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 " .
also referenced under: Book 46, Hadith 44 (In-Book) & Hadith 2661 (Reference)