Given to the army before the encounter with the enemy at Siffin 1
ومن وصيّته (عليه السلام)
لعسكره قبل لقاء العدو بصفّين
Do not fight them unless they initiate the fighting, because, by the grace of Allah, you are in the right and to leave them till they begin fighting will be another point from your side against them. If, by the will of Allah, the enemy is defeated then do not kill the runner away, do not strike a helpless person, do not finish off the wounded. Do not inflict pain on women even though they may attack your honour with filthy words and abuse your officers, because they are weak in character, mind and intelligence. We have been ordered to desist from them although they may be unbelievers. Even in the pre-Islamic (al-jahiliyyah) period if a man struck a woman with a stone or a stick he was rebuked along with his offspring after him.
لاَ تُقَاتِلُوهُمْ حَتَّى يَبْدَأُوكُمْ، فَإِنَّكُمْ بِحَمْدِ اللهِ عَلَى حُجَّة، وَتَرْكُكُمْ إِيَّاهُمْ حَتَّى يَبْدَأُوكُمْ حُجَّةٌ أُخْرَى لَكُمْ عَلَيْهِمْ، فَإذَا كَانَتِ الْهَزِيمَةُ بِإذْنِ اللهِ فَلاَ تَقْتُلُوا مُدْبِراً، وَلاَ تُصيِبُوا مُعْوِراً، وَلاَ تُجْهِزُوا عَلَى جَرِيح،لاَ تَهِيجُوا النِّسَاءَ بِأَذىً، وَإِنْ شَتَمْنَ أَعْرَاضَكُمْ، وَسَبَبْنَ أُمَرَاءَكُمْ، فَإِنَّهُنَّ ضَعِيفَاتُ الْقُوَى وَالاْنْفُسِ وَالْعُقُولِ، إِنْ كُنَّا لَنُؤْمَرُ بِالْكَفِّ عَنْهُنَّ وَإِنَّهُنَّ لَمُشْرِكَاتٌ، وَإِنْ كَانَ الرَّجُلُ لَيَتَنَاوَلُ الْمَرْأَةَ فِي الْجَاهِلِيَّةِ بِالْفِهْرِ أَوِ الْهِرَاوَةِ فَيُعَيَّرُ بِهَا وَعَقِبُهُ مِنْ بَعْدِهِ.
Muhammad ibn 'Abd al-Karim ash- Shahrastani writes:
Whoever rises against the true Imam, by the unanimity of opinion of the (Muslim) community, is known as a Kharijite, the deviator. The same is the case of rising, during the days of the companions, against the rightful Imams, or even after them against those who followed them in virtue. (al-Milal wa'n-nihal, vol.1, p.114)
There is no doubt that Mu'awiyah's action was the result of uprising and revolt, and to take up arms for the purpose of stopping the advance of the one who revolts is not to be regarded as being against any code of peacefulness or peace-loving. Rather, it is a natural right of the oppressed; and if he is deprived of this right then there will remain no way of preventing oppression and tyranny or of safeguarding rights in the world. That is why Allah has permitted taking up arms against rebels. Thus, Allah says:
And if two parties of the believers fall into a quarrel (among themselves), restore ye peace between them two; but if one of the two transgresseth against the other, (then) fight ye (all against) that which transgresseth until it complieth with the command of Allah; and if it complieth then restore ye peace between the two with justice, and act ye justly; Verily, Allah loveth the just ones. (Qur'an, 49:9)
It was the first plea to which Amir al-mu'minin pointed out by saying, "By the Grace of Allah you are in the right," but even after exhaustion of this plea he prevented his army from taking the initiative in fighting, because he wished that the initiative should not be from his side and that he should take up the sword only in defence. Consequently, when all his effort for peace and tranquility proved futile and the enemy took the step towards war, this was the second argument in their favour, after which Amir al-mu'minin could not be blamed for getting ready to fight, nor accused of aggressive action. It was rather an obligation to stop oppression and tyranny that he had to discharge and which Allah has permitted in plain words. Thus, Allah's command is that:
And fight in the cause of Allah (against) those who fight you but be not aggressive; for verily Allah loveth not the aggressors. (Qu'an, 2:190)
Besides, fighting against Amir al-mu'minin means fighting against the Prophet, as the Prophet's saying: "O' 'Ali, your peace is my peace and your war is my war" (Ibn al-Maghazili, al-Manaqib, p.5; Ibn Abi'l-Hadid, vol.18, p.24). In this way whatever punishment should be for fighting against the Prophet should be for fighting against Amir al-mu'minin. For him who wages war against the Prophet, Allah has laid down the following punishment.
To recompense of those who war against Allah and His Apostle, and strive in the land, spreading mischief (therein), is only that they be slain or crucified or their hands and their feet should be cut off, from the opposite sides, or be banished from the land; This for them shall be the disgrace in this world, and for them, in the hereafter, shall be a great torment. (Qur'an, 5:33)
Apart from this, the instructions that Amir al-mu'minin issued in connection with the war, namely that no runner away or wounded should be molested, are so high from the moral point of view that they can be regarded as a sublime specimen of moral values and the high standard of Islamic fighting. Then, these instructions were not confined to mere words but Amir al-mu'minin followed them to the letter, and ordered others also to follow them strictly. He did not, on any occasion, tolerate the chasing of a runner away, attack the helpless or molest women, in fact, on the battlefield of Jamal, where the command of the opposite force was with a woman, he did not change his principle. After the defeat and vanquishment of the foe he gave proof of his high character and sent 'A'ishah to Medina under guard. Had there been someone other than Amir al-mu'minin he would have proposed the same punishment as that which ought to be awarded for such a step! Thus, Ibn Abi'l-Hadid has written:
What she did with Amir al-mu'minin, if she had done the same with (Caliph) 'Umar and had spread rebellion against him among the people, he would , after securing victory over her, have killed her and cut her into pieces, but Amir al-mu'minin was very forbearing and large-hearted .(Sharh Nahjul Balaghah al-balaghah, vol.17, p.254)