Sahih Muslim Book - 32, Hadith - 4685

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِى 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 فِى حَدِيثِ حَاتِمٍ وَإِنَّ رَسُولَ اللَّهِ -صلى الله عليه وسلم- لَمْ يَكُنْ يَقْتُلُ الصِّبْيَانَ فَلاَ تَقْتُلِ الصِّبْيَانَ إِلاَّ أَنْ تَكُونَ تَعْلَمُ مَا عَلِمَ الْخَضِرُ مِنَ الصَّبِىِّ الَّذِى قَتَلَ. وَزَادَ إِسْحَاقُ فِى حَدِيثِهِ عَنْ حَاتِمٍ وَتُمَيِّزَ الْمُؤْمِنَ فَتَقْتُلَ الْكَافِرَ وَتَدَعَ الْمُؤْمِنَ.

It was narrated from Yazid bin Hurmuz that Najdah wrote to Ibn 'Abbas and asked him about some things... a Hadith like that of Sulaiman bin Bilal (no. 4684), except that in the Hadith of Hatim it says: "The Messenger of Allah (s.a.w) did not kill children, so do not kill children, unless you know what Al-Khidr knew about the boy whom he killed." Ishaq added in his Hadith from Hatim: "...and you can tell who is a believer, in which case kill the disbelievers and leave the believers."