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

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.