Hadith: Whenever the Prophet (may Allah's peace and blessings be upon him) appointed a man to lead an army or a military detachment, he would advise him to fear Allah and treat well those Muslims who are with him

عن بُرَيْدَة بن الحُصَيب الأَسْلَمِيّ رضي الله عنه قال: كان رسول الله صلى الله عليه وسلم إذا أمَّر أميرًا على جَيْش أو سَرِيَّة أَوْصَاه بتَقْوَى الله، ومَن معه مِن المسلمين خيرًا، فقال: "اغْزُوا بسم الله في سبيل الله، قاتِلُوا مَن كَفَر بالله، اغْزُوا ولا تَغُلُّوا ولا تَغْدِروا ولا تُـمَثِّلُوا ولا تَقْتُلُوا وَلِيدًا، وإذا لَقِيتَ عَدُوَّك مِن المشركين فادْعُهم إلى ثلاث خِصال -أو خِلال-، فأيَّتُهُنَّ ما أجابوك فاقْبَلْ منهم وكُفَّ عنهم، ثم ادْعُهم إلى الإسلام فإن أجابوك فاقْبَلْ منهم. ثم ادْعُهم إلى التَّحَوُّل مِن دارهم إلى دار المهاجرين، وأَخْبِرْهم أنهم إن فَعَلُوا ذلك فلهم ما للمهاجرين وعليهم ما على المهاجرين، فإن أَبَوْا أن يَتَحَوَّلُوا منها فأَخْبِرْهم أنهم يكونون كأَعْرَاب المسلمين يَجْرِي عليهم حُكْمُ الله تعالى، ولا يكون لهم في الغَنِيمَة والفَيْء شيءٌ إلا أن يُجَاهِدُوا مع المسلمين، فإن هم أَبَوْا فاسْأَلْهم الجِزْيَةَ، فإن هم أجابوك فاقْبَلْ منهم وكُفَّ عنهم، فإن هم أَبَوْا فاستَعِن بالله وقَاتِلْهم. وإذا حاصَرْتَ أَهْلَ حِصْنٍ فأرادُوك أن تَجْعَلَ لهم ذِمَّةَ الله وذِمَّةَ نَبِيِّه، فلا تَجْعَلْ لهم ذِمَّةَ الله وذِمَّةَ نَبِيِّه، ولكن اجْعَلْ لهم ذِمَّتَك وذِمَّةَ أصحابك؛ فإنكم أن تُخْفِرُوا ذِمَمَكم وذِمَّةَ أصحابكم أَهْوَنُ مِن أن تُخْفِرُوا ذِمَّةَ الله وذِمَّةَ نَبِيِّه، وإذا حاصَرْتَ أَهْلَ حِصْنٍ فأرادُوك أن تُنْزِلَهم على حُكْم الله فلا تُنْزِلْهم، ولكن أَنْزِلْهم على حُكْمِك، فإنك لا تَدْرِي أَتُصِيبُ فيهم حُكْمَ الله أم لا".
[صحيح] - [رواه مسلم]

Buraydah ibn al-Husayb al-Aslami (may Allah be pleased with him) reported: Whenever the Prophet (may Allah's peace and blessings be upon him) appointed a man to lead an army or a military detachment, he would advise him to fear Allah and treat well those Muslims who are with him. He would say: "Fight in the name of Allah and in the cause of Allah. Fight those who disbelieve in Allah. Fight, but do not steal from the spoils of war, do not be treacherous, do not mutilate, and do not kill children. When you meet your enemy from among the polytheists, call them to one of three things. Whichever of them they respond to, accept it from them and refrain from fighting them. Invite them to accept Islam, and if they respond, accept it from them and refrain from fighting them. Then invite them to leave their land and move to the land of the Emigrants (Madīnah). Tell them that if they do so, they will have the same rights and duties as the Emigrants. If they refuse to move, tell them they will be like the Muslim Bedouins, subject to the rulings of Allah, and they will have no share in war booty or spoils of war, unless they fight alongside the Muslims. If they refuse to enter Islam, ask them to pay the Jjizyah (poll tax paid by non-Muslims). If they do so, accept it from them and refrain from fighting them. But if they refuse, seek the help of Allah against them and fight them. If you besiege a group and they want you to give them the covenant of Allah and His Prophet, do not give them the covenant of Allah and His Prophet, instead give them your own covenant and the covenant of your companions, for if you violate your covenant and the covenant of your companions, that is easier than violating the covenant of Allah and the covenants of His Prophet. If you besiege a group of people and they want you to apply the judgment of Allah upon them, do not offer them that, but rather offer them your own judgment, because you do not know whether you would apply correctly the judgment of Allah upon them."
Sahih/Authentic. - [Muslim]

Explanation

Buraydah (may Allah be pleased with him) reported that whenever the Prophet (may Allah's peace and blessings be upon him) would send an army or a military detachment to fight the disbelievers, he would appoint a leader or emir to them to preserve their unity and take care of their affairs. He would advise him to be righteous and to take good care of those with him. Then he would guide him to what they should do with their enemies: they must avoid stealing from the spoils of war and booty, avoid treachery and breaking covenants, avoid mutilating the bodies, and avoid killing those under the age of puberty. They should start by calling the polytheists to Islam, and if they accept that, they should urge them to emigrate to Madīnah, telling them that they if they do, they will have the same rights and the responsibilities as those who emigrated before them. But, if they refuse to emigrate, they will be treated like the bedouin Muslims. On the other hand, if they refuse to embrace Islam, the Muslims should ask them to pay the Jizyah instead. If they refuse to pay the Jizyah, the Muslims should seek help of Allah and start fighting them. If Muslims besieged people in a castle, they were told not give them a covenant or promise in the name of Allah or His Messenger, but instead give their own covenant or promise, because if the covenant is broken, it would be less sinful to break someone’s covenant than to break that of Allah and His Messenger. Likewise, if besieged people request Muslims to apply the judgment of Allah upon them, Muslims should not implement a specific judgment and ascribe it to Allah, as they might not truly implement the exact judgment of Allah. Instead, they should implement their own best and reasonable judgment.