Sayyiduna Abu Musa (may Allah be pleased with him) says: When the Messenger of Allah (peace and blessings be upon him) would send a military expedition, he would say: "Fight in the name of Allah, and fight in the way of Allah, fight against the one who disbelieves in Allah. Do not be treacherous, do not deceive, do not mutilate, and do not kill any child or elderly person."
Hadith Referenceمعجم صغير للطبراني / كتاب الجهاد / 578
Hadith Gradingمحدثین:إسناده صحيح
Hadith Takhrij«إسناده صحيح ، أخرجه الطبراني فى «الصغير» برقم: 514، والبزار فى «مسنده» برقم: 3122، وأما حديث أنس بن مالك أخرجه البخاري فى «صحيحه» برقم: 4192، وأبو داود فى «سننه» برقم: 2614، والنسائي فى «المجتبیٰ» برقم: 4058 ، والبيهقي فى«سننه الكبير» برقم: 17402»
Narrated Anas ibn Malik: The Prophet ﷺ said: Go in Allah's name, trusting in Allah, and adhering to the religion of Allah's Messenger. Do not kill a decrepit old man, o a young infant, or a child, or a woman; do not be dishonest about booty, but collect your spoils, do right and act well, for Allah loves those who do well.
Shaykh Umar Farooq Saeedi
Benefits and Issues: It is not permissible to kill an elderly person in battle. Except for such elderly people about whom it is known that they give plans and strategies. And such women who are involved in matters like espionage, etc. It will be permissible to kill them.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 2614