Sahih Muslim #4685 · The Book of Jihad and Expeditions

Sahih MuslimThis tradition has been narrated by the game authority
Sahih Muslim · Book 32 · The Book of Jihad and Expeditions
Sahih Muslim · كتاب 32 · كتاب الجهاد والسير
The Messenger of Allah (ﷺ) used not to kill the children, so thou shouldst not kill them unless you could know what Khadir had known about the child he killed, or you could distinguish between a child who would grow up to he a believer (and a child who would grow up to be a non-believer), so that you killed the (prospective) non-believer and left the (prospective) believer aside.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ، كِلاَهُمَا عَنْ حَاتِمِ بْنِ إِسْمَاعِيلَ، عَنْ جَعْفَرِ بْنِ مُحَمَّدٍ، عَنْ أَبِيهِ، عَنْ يَزِيدَ بْنِ هُرْمُزَ، أَنَّ نَجْدَةَ، كَتَبَ إِلَى ابْنِ عَبَّاسٍ يَسْأَلُهُ عَنْ خِلاَلٍ، ‏.‏ بِمِثْلِ حَدِيثِ سُلَيْمَانَ بْنِ بِلاَلٍ غَيْرَ أَنَّ فِي، حَدِيثِ حَاتِمٍ وَإِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم لَمْ يَكُنْ يَقْتُلُ الصِّبْيَانَ فَلاَ تَقْتُلِ الصِّبْيَانَ إِلاَّ أَنْ تَكُونَ تَعْلَمُ مَا عَلِمَ الْخَضِرُ مِنَ الصَّبِيِّ الَّذِي قَتَلَ ‏.‏ وَزَادَ إِسْحَاقُ فِي حَدِيثِهِ عَنْ حَاتِمٍ وَتُمَيِّزَ الْمُؤْمِنَ فَتَقْتُلَ الْكَافِرَ وَتَدَعَ الْمُؤْمِنَ ‏.‏
Reference : Sahih Muslim 4685In-book reference : Book 32, Hadith 167