Sunan an-Nasa'i, Hadith 1949 ⓘ
It was narrated that Abu Hurairah said: "The Messenger of Allah was asked about the children of the idolators and he said: 'Allah knows best what they would have done."'
أَخْبَرَنَا إِسْحَاقُ، قَالَ أَنْبَأَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ عَطَاءِ بْنِ يَزِيدَ اللَّيْثِيِّ، عَنْ أَبِي هُرَيْرَةَ، قَالَ سُئِلَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ أَوْلاَدِ الْمُشْرِكِينَ فَقَالَ " اللَّهُ أَعْلَمُ بِمَا كَانُوا عَامِلِينَ " .
also referenced under: Book 21, Hadith 133 (In-Book) & Volume 3, Book 21, Hadith 1951 (DarusSalam)