Aswad ibn Sari’ reported: I came with the Messenger of Allah, peace and blessings be upon him, and participated in the battle with him. I engaged in the skirmish that day, until some people killed children. The Prophet was told about it and he said, “What is wrong with these people transgressing the limits of killing, even having killed children?” A man said, “O Messenger of Allah, they are only the children of idolaters.” The Prophet said, “No doubt, the best of you are the children of idolaters.” Then, the Prophet said, “No doubt, do not kill children! Do not kill children! Every soul is born upon its natural instinct until he expresses it with his tongue, then his father turns him into a Jew or a Christian.”
Source: Musnad Aḥmad 15589
Grade: Sahih (authentic) according to Al-Albani
عَنِ الْأَسْوَدِ بْنِ سَرِيعٍ قَالَ أَتَيْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَغَزَوْتُ مَعَهُ فَأَصَبْتُ ظَهْرًا فَقَتَلَ النَّاسُ يَوْمَئِذٍ حَتَّى قَتَلُوا الْوِلْدَانَ وَقَالَ مَرَّةً الذُّرِّيَّةَ فَبَلَغَ ذَلِكَ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ مَا بَالُ أَقْوَامٍ جَاوَزَهُمْ الْقَتْلُ الْيَوْمَ حَتَّى قَتَلُوا الذُّرِّيَّةَ فَقَالَ رَجُلٌ يَا رَسُولَ اللَّهِ إِنَّمَا هُمْ أَوْلَادُ الْمُشْرِكِينَ فَقَالَ أَلَا إِنَّ خِيَارَكُمْ أَبْنَاءُ الْمُشْرِكِينَ ثُمَّ قَالَ أَلَا لَا تَقْتُلُوا ذُرِّيَّةً أَلَا لَا تَقْتُلُوا ذُرِّيَّةً قَالَ كُلُّ نَسَمَةٍ تُولَدُ عَلَى الْفِطْرَةِ حَتَّى يُعْرِبَ عَنْهَا لِسَانُهَا فَأَبَوَاهَا يُهَوِّدَانِهَا وَيُنَصِّرَانِهَا
15589 مسند أحمد بن حنبل
المحدث الألباني خلاصة حكم المحدث صحيح على شرط الشيخين في السلسلة الصحيحة 402