(6) بَاب مَعْنَى كُلِّ مَوْلُودٍ يُولَدُ عَلَى الْفِطْرَةِ وَحُكْمِ مَوْتِ أَطْفَالِ الْكُفَّارِ وَأَطْفَالِ الْمُسْلِمِينَ
Abu Huraira reported that Allahs Messenger (way peace be upon him) was asked about the children of the polytheists who die young. Thereupon Allah's Apostle (ﷺ) said:
It is Allah Who knows what they would be doing.
Reference: Sahih Muslim 2660
In-book reference: Book 46, Hadith 43
USC-MSA web (English) reference(deprecated numbering scheme): Book 33, Hadith 6432