In one of the days of Siffin Amir al-mu'minin said to his followers about ways of fighting
ومن كلام له (عليه السلام)
في تعليم الحرب والمقاتلة
في بعض أيام صفين
O Muslims! Make fear of Allah the routine of your life. Cover yourselves with peace of mind and clench your teeth because this makes the sword slip off from the skull. Complete your armour and shake your swords in their sheathes before showing them out. Have your eyes on the enemy. Use your spears on both sides and strike (the enemy) with swords. Keep in mind that you are before Allah and in the company of the Prophet's cousin.

Repeat your attacks and feel ashamed of running away, because it is a shame for posterity and (cause of awarding you) fire on the Day of Judgement. Give your lives (to Allah) willingly and walk towards death with ease. Beware of this great majority, and the pitched tent and aim at its centre because Satan is hiding in its corner. He has extended his hand for assault and has kept back his foot for running away. Keep on enduring till the light of Truth dawns upon you.

While ye have the upper hand, and Allah is with you, and never will He depreciate your deeds. (Qur'an, 47:35)

مَعَاشِرَ الْمُسْلِمِينَ: اسْتَشْعِرُوا الْخَشْيَةَ وَتَجَلْبَبُواالسَّكِينَةَ، وَعَضُّواعَلَى النَّوَاجِذِ فَإِنَّهُ أَنْبَى لِلْسُّيُوفِ عَنِ الْهَامِ وَأَكْمِلُوا اللاَّمَةَ ،وَقَلْقِلُواالسُّيُوفَ فِي أَغْمَادِهَا قَبْلَ سَلِّهَا، وَالْحَظُوا الْخَزْرَ وَاطْعُنُوا الشَّزْرَ وَنَافِحُوا بِالظُّبَا وَصِلُوا السُّيُوفَ بَالْخُطَا وَاعْلَمُوا أَنَّكُمْ بِعَيْنِ اللهِ، وَمَعَ ابْنِ عَمِّ رَسُولِ اللهِ صلى الله عليه وآله ، فَعَاوِدُوا الْكَرَّ، وَاسْتَحْيُوا مِنَ الْفَرِّ فَإِنَّهُ عَارٌ فِي الاْعْقَابِ وَنَارٌ يَوْمَ الْحِسَابِ، وَطِيبُوا عَنْ أَنْفُسِكُمْ نَفْساً، وَامْشُوا إِلَى الْمَوْتِ مَشْياً سُجُحاً وَعَلَيْكُمْ بِهذَا السَّوَادِ الاْعْظَمِ، وَالرِّوَاقِ المُطَنَّبِ، فَاضْرِبُوا ثَبَجَهُ فإِنَّ الشَّيْطَانَ كَامِنٌ فِي كِسْرِهِ قَدْ قَدَّمَ لِلْوَثْبَةِ يَداً، وَأَخَّرَ لِلنُّكُوصِ رِجْلاً؛ فَصَمْداً صَمْداً! حَتَّى يَنْجَلِيَ لَكُمْ عَمُودُ الْحَقِّ . (وَأَنْتُمُ الاْعْلَوْنَ وَاللهُ مَعَكُمْ وَلَنْ يَتِرَكُمْ أَعْمَالَكُمْ).