The Book of Destiny
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 " .Ibn Abbas reported that Allah's Messenger (ﷺ) 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
From Sahih Muslim, whose reports the scholars accept as authentic.