كتاب القدر
46  
The Book of Destiny
(6)
Chapter: 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.
(6)
بَاب مَعْنَى كُلِّ مَوْلُودٍ يُولَدُ عَلَى الْفِطْرَةِ وَحُكْمِ مَوْتِ أَطْفَالِ الْكُفَّارِ وَأَطْفَالِ الْمُسْلِمِينَ
Sahih Muslim 2661

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.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 2661
In-book reference : Book 46, Hadith 44
USC-MSA web (English) reference : Book 33, Hadith 6433
  (deprecated numbering scheme)