Abu Dawud

Buraidah (may Allah be pleased with him) narrates that whenever the Messenger of Allah (peace and blessings of Allah be upon him) appointed a commander over an army, he would especially advise him to fear Allah regarding his own self and to do good to the Muslims who were with him. After that, he would say: Fight in the name of Allah and in His path, fight those who disbelieve in Allah, do not embezzle from the spoils of war, do not break covenants, do not mutilate, do not kill children. And when you face your polytheist enemies, invite them to three things; whichever of them they accept, accept it from them and refrain from fighting them: Invite them to Islam and to migrate from their homeland to the land of the emigrants, and inform them that if they do so, they will have the same rights as the emigrants and the same responsibilities as the emigrants. But if they refuse to migrate, inform them that they will be like the Bedouin Muslims, and the same rulings will apply to them as apply to the Bedouin Muslims: they will have no share in the spoils of war or in the fay’ unless they fight alongside the Muslims. If they refuse to do this, then seek Allah’s help against them and begin to fight them. When you besiege a fortress and they want you to grant them protection in the name of Allah and His Prophet, do not grant them the protection of Allah and His Prophet, but grant them your own and your companions’ protection (and do not break it), for if you break your own and your companions’ protection, it is less serious than breaking the protection of Allah and His Messenger. And when you besiege the people of a fortress and they want you to refer their case to the judgment of Allah, do not refer it to the judgment of Allah, but refer it to your own judgment, for you do not know whether you will be able to reach the judgment of Allah concerning them or not.” He also said some other things in this regard __QUR_N__, __XREF_N__, __REF_N__.

Imam Tirmidhi says:
1- The hadith of Buraidah is Hasan Sahih (good and authentic),
2- In this chapter, there is also a narration from Nu’man bin Muqarrin (may Allah be pleased with him).

´Narrated Buraidah:` from his father who said: "Whenever the Messenger of Allah (ﷺ) dispatched a commander of an army he would exhort him personally; that he should have Taqwa of Allah, and regarding those of the Muslims who are with him; that he should be good to them. He would say: 'Fight in the Name of Allah and in Allah's curse. Fight those who disbelieve in Allah and fight, do not be treacherous, nor mutilate, nor kill a child."