Imam Ahmad about what happened between Ali and Muawiyah

A man asked Imam Ahmad ibn Hanbal (rahimahullaah) about what happened between ‘Ali and Mu‘aawiyah (May Allaah be pleased with them), and he turned away from him.
It was said to him: O Abu ‘Abdullah, he is a man from Banu Haashim. He turned to him and said: Recite: “That was a nation who has passed away. They shall receive the reward of what they earned and you of what you earn. And you will not be asked of what they used to do” [al-Baqarah 2:134].
[Source: Manaaqib al-Imam Ahmad by Ibn al-Jawzi, p. 126]
Imam Ahmad also said, after it was said to him: What do you say about what happened between ‘Ali and Mu‘aawiyah (May Allaah be pleased with them)? He said: I do not say anything about them except what is best.
[Source: Manaaqib al-Imam Ahmad by Ibn al-Jawzi, p. 164]
What Imaam Ahmad (rahimahullaah) stated is one of the most important principles of Ahl as-Sunnah wa’l-Jamaa‘ah, which distinguishes them from the Raafidah (Shias) and the Ahlul Bid’ah.
Al-Maymooni said: Ahmad ibn Hanbal (rahimahullaah) said to me: O Abu’l-Hasan, if you see a man saying anything bad about any one of the Sahaabah, then be suspicious about his Islam.
Al-Fadl ibn Ziyaad said: I heard Abu ‘Abdullah being asked about a man who criticized Mu‘aawiyah and ‘Amr ibn al-‘Aas (May Allaah be pleased with them): can he be called a Raafidi? He said: He did not have the audacity to criticise them except because he has some evil hidden in his heart. No one criticises any one of the Sahaabah but he is concealing some evil in his heart.
[Source: Al-Bidaayah wa’n-Nihaayah, 8/139]
With regard to what happened among the Sahaabah of differences of opinion and fighting, we should refrain from discussing it, whilst believing that they are the best of the ummah and loving them and approving of them. This opinion has been repeatedly stated by Ahl as-Sunnah wa’l-Jamaa‘ah. [Also read: The hearts and tongues of Ahl-ul-Sunnah are free of any hatred towards the Sahabah]
‘Umar ibn ‘Abd al-‘Azeez (rahimahullaah) was asked about ‘Ali and ‘Uthmaan (May Allaah be pleased with them), and the battles of the Camel and Siffeen, and what happened among them. He said: That is blood that Allah caused me to have no hand in shedding, and I do not want to dip my tongue in it (by talking about it).
[Source: at-Tabaqaat al-Kubra, 5/394]
Abu Zar‘ah ar-Raazi said:
If you see a man criticising any of the Companions of the Messenger of Allah (blessings and peace of Allah be upon him), then know that he is a heretic, because in our view the Messenger (blessings and peace of Allah be upon him) is true and the Qur’aan is true. Rather this Qur’aan and the Sunnah were conveyed to us by the Companions of the Messenger of Allah (blessings and peace of Allah be upon him). They only want to undermine the status of those who conveyed the Qur’aan (and the Sunnah) in order to undermine the validity of the Qur’aan and Sunnah. But it is more fitting that they be criticised, and they are heretics.
[Source: Al-Kifaayah fi ‘Ilm ar-Riwaayah, p. 49]
Abu ‘Abdullah ibn Battah (may Allah have mercy on him) said, discussing the beliefs of Ahl as-Sunnah wa’l-Jamaa‘ah:
After that we should refrain from discussing the disputes that arose among the Companions of the Messenger of Allah (blessings and peace of Allah be upon him), for they witnessed great events with him and were the first people to attain virtue; Allah has forgiven them and has instructed us to pray for forgiveness for them and to draw close to Him by means of loving them, as He has enjoined on the lips of His Prophet. He knew what would happen among them and that they would fight; however they were given precedence over the rest of mankind because mistakes, whether deliberate or otherwise, were already forgiven for them, and they have been forgiven for all disputes that arose among them.
[Source: Kitaab ash-Sharh wa’l-Ibaanah ‘ala Usool as-Sunnah wa’d-Diyaanah, p. 268]
Abu ‘Uthmaan as-Saaboni said, discussing the belief of the Salaf and scholars of hadeeth:
They believed that we should refrain from discussing the disputes that arose among the Companions of the Messenger of Allah (blessings and peace of Allah be upon him) and that we should avoid saying anything that may be construed as criticism of them; we should pray for mercy for all of them and love all of them.
[Source: ‘Aqeedat as-Salaf wa Ashaab al-Hadeeth, in Majmoo‘at ar-Rasaa’il al-Muneeriyyah, 1/129]
A great deal of similar comments have been made by the scholars and it is essential to follow that, because this will protect us from making mistakes and it is acknowledging the rights of the best of people after the Prophets.
As for the killing of ‘Uthmaan, ‘Ali and Hussain ibn Ali (may Allah be pleased with them all), there is no doubt that they were killed unlawfully and we testify that they will be in Paradise, and we testify that the one who did this action, or played any part in it, or approved of it, or knew of it and did not denounce it, were led astray in the worst of ways and led others astray, and was a man of evil, who followed a path other than that of the believers; Allah will bring him to account on the Day of Resurrection for his actions and Allah, may He be exalted, will judge between them on the Day of Resurrection. “Say (O Muhammad (sallallahu ‘alayhi wasallam)): O Allah! Creator of the heavens and the earth! All-Knower of the Ghaib (unseen) and the seen. You will judge between your slaves about that wherein they used to differ” [az-Zumar 39:46].