Hadith: that a woman was found killed in one of the expeditions of the Prophet (may Allah's peace and blessings be upon him), and so the Messenger of Allah (may Allah's peace and blessings be upon him) denounced killing women and boys

عَنْ عَبْدِ اللَّهِ بنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُما:
أَنَّ امْرَأَةً وُجِدَتْ فِي بَعْضِ مَغَازِي النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ مَقْتُولَةً، فَأَنْكَرَ رَسُولُ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَتْلَ النِّسَاءِ وَالصِّبْيَانِ.
[صحيح] - [متفق عليه] - [صحيح البخاري: 3014]

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported
that a woman was found killed in one of the expeditions of the Prophet (may Allah's peace and blessings be upon him), and so the Messenger of Allah (may Allah's peace and blessings be upon him) denounced killing women and boys.
[Authentic] - [Agreed upon] - [Sahīh Al-Bukhāri - 3014]

Explanation

The Prophet (may Allah's peace and blessings be upon him) saw a woman killed in one of the battles, and he denounced killing women and young boys who had not reached puberty.

Benefits from the Hadith

1. Those who do not fight, like women and boys and the elderly and monks, should not be killed. An exception is when they advocate and help the fighting against Muslims, in which case they can be killed.
2. It is prohibited to kill women and boys, for those who do not fight Muslims. The purpose of Jihad in the cause of Allah is to subdue fighters only, so that the Da‘wah of the truth can reach all people.
3. The Hadīth demonstrates the Prophet's mercy even during battles and wars.