Muwatta Malik 967

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم رَأَى فِي بَعْضِ مَغَازِيهِ امْرَأَةً مَقْتُولَةً فَأَنْكَرَ ذَلِكَ وَنَهَى عَنْ قَتْلِ النِّسَاءِ وَالصِّبْيَانِ ‏.‏

Yahya related to me from Malik from Nafi from Ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, saw the corpse of a woman who had been slain in one of the raids, and he disapproved of it and forbade the killing of women and children

Reference

Muwatta Malik 967

In-book

Book 21, Hadith 9