Bukhari

Hadith Commentary:
What happened was that during the Battle of Siffin, when the people of Ali began to gain the upper hand over the people of Muawiyah, Amr ibn al-As radi Allahu anhu advised Muawiyah to send the Noble Qur’an to Ali and say, “Let us both act upon this.”
Ali would certainly agree to act upon the Noble Qur’an.
When the Noble Qur’an was brought forth, Ali said, “I am even more committed to acting upon it than you.”
At that moment, the Kharijites arrived, who were called the Qurra (reciters), and they said, “O Commander of the Faithful! We will not wait; we are going to fight them, we will fight them.”
The Kharijites said, “We will not accept arbitration (tahkim), because no one can be a judge except Allah.”
Let there be fighting, and let one side prevail.
Suhayl ibn Hunayf’s speech was against the Kharijites, as is mentioned in the narration. The commentators write:

His statement: “Sahl ibn Hunayf said, ‘Accuse yourselves, for I do not fall short, nor was I negligent when there was a need, as on the Day of Hudaybiyyah. For I saw myself that day in such a state that, had I been able to oppose the Messenger of Allah sallallahu alayhi wa sallam, I would have fought a great fight. But today, we do not see any benefit in fighting; rather, refraining is for the benefit of the Muslims. As for objecting to arbitration, there is nothing in the Book of Allah that prohibits it.’ Ali said, ‘Yes, but those who object among them are those who have deviated from the Book of Allah, because when the mujtahid’s ijtihad leads him to the permissibility of arbitration, then that is the ruling of Allah.’ And Sahl said, ‘You accused yourselves in your objection, for we too disliked abandoning fighting on the Day of Hudaybiyyah, and the Prophet sallallahu alayhi wa sallam compelled us to make peace, and it resulted in much good.’”
(Kirmani)

Hadith Commentary:
➊
The events are as follows: During the Battle of Siffin, when the soldiers of Ali radi Allahu anhu began to gain the upper hand over the soldiers of Amir Muawiyah radi Allahu anhu, Amr ibn al-As radi Allahu anhu advised Amir Muawiyah radi Allahu anhu to raise the Qur’an and say that both parties should let the Qur’an decide between them.
When the Qur’an was brought forth, Ali radi Allahu anhu said:
“I am the one among you who acts upon it the most.”
At that moment, the Khawarij flared up and began to say that they did not accept this decision, because the decision should be Allah’s alone; they would not wait for this verdict.
They insisted that they would certainly fight.
In this situation, Sahl ibn Hunayf radi Allahu anhu delivered a speech against the Khawarij.
He said that at the time of the Treaty of Hudaybiyyah, such conditions were presented to them that they were not ready to accept them, rather, they were prepared to fight and be killed. But they controlled their emotions and reconsidered their opinion, and in this, Allah bestowed goodness and blessing.

➋
In any case, the Treaty of Hudaybiyyah, which the Muslims apparently considered their humiliation and defeat, was in reality their miraculous victory, the reality of which was not understood even by the Muslims themselves, let alone the disbelievers. Even Umar radi Allahu anhu could not control his emotions at that time. Then, as its results gradually became apparent, the Muslims became increasingly convinced that indeed this treaty was one of the signs of Allah.
Umar radi Allahu anhu realized this; he himself states that, in order to make amends for the disrespectful words he uttered on that occasion, he performed many good deeds afterwards.
(Sahih al-Bukhari, al-Shurut, Hadith: 2731)