Narrated Al-Hasan Al-Basri (raa): By Allah, Al-Hasan bin Ali (raa) led large battalions like mountains against Muawiya (raa). Amr bin Al-As said (to Muawiya (raa)), "I surely see battalions which will not turn back before killing their opponents."
Muawiya (raa) who was really the best of the two men said to him, "O ‘Amr! If these killed those and those killed these, who would be left with me for the jobs of the public, who would be left with me for their women, who would be left with me for their children?"
Then Muawiya (raa) sent two Quraishi men from the tribe of ‘Abd-i-Shams called ‘Abdur Rahman bin Sumura and Abdullah bin ‘Amir bin Kuraiz to Al-Hasan saying to them, "Go to this man (i.e. Al-Hasan) and negotiate peace with him and talk and appeal to him."
So, they went to Al-Hasan and talked and appealed to him to accept peace.
Al-Hasan said, "We, the offspring of ‘Abdul Muttalib, have got wealth and people have indulged in killing and corruption (and money only will appease them)."
They said to Al-Hasan, "Muawiya offers you so and so, and appeals to you and entreats you to accept peace."
Al-Hasan said to them, "But who will be responsible for what you have said?"
They said, "We will be responsible for it."
So, what-ever Al-Hasan asked they said, "We will be responsible for it for you."
So, Al-Hasan concluded a peace treaty with Muawiya (raa).
Al-Hasan (Al-Basri) said: I heard Abu Bakr (raa) saying, "I saw Allah’s Apostle (saws) on the pulpit and Al-Hasan bin ‘Ali was by his side. The Prophet (saws) was looking once at the people and once at Al-Hasan bin ‘Ali saying, ‘This son of mine is a Saiyid (i.e. a noble) and may Allah make peace between two big groups of Muslims through him."
Peacemaking – Sahih Bukhari: Volume 3, Book 49, Number 867