Sahih Muslim 1744
Sahih Muslim · Book 32: The Book of Jihad and Expeditions · The prohibition of killing women and children in war
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا مُحَمَّدُ بْنُ بِشْرٍ ، وَأَبُو أُسَامَةَ قَالاَ حَدَّثَنَا عُبَيْدُ، اللَّهِ بْنُ عُمَرَ عَنْ نَافِعٍ ، عَنِ ابْنِ عُمَرَ ، قَالَ وُجِدَتِ امْرَأَةٌ مَقْتُولَةً فِي بَعْضِ تِلْكَ الْمَغَازِي فَنَهَى رَسُولُ اللَّهِ صلى الله عليه وسلم عَنْ قَتْلِ النِّسَاءِ وَالصِّبْيَانِ .
It is narrated by Ibn 'Umar that a woman was found killed in one of these battles; so the Messenger of Allah (ﷺ) forbade the killing of women and children.