Who is the person about whom the Prophet, sallallaahu 'alayhi wa sallam, said the following: "Verily, this son of mine is a Sayyid (chief), and Allaah will reconcile between two great groups of Muslims by him"? What is the story of what the Prophet, sallallaahu 'alayhi wa sallam, is speaking about?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Abu Bakrah said, “I saw the Messenger of Allah, sallallaahu ‘alayhi wa sallam, on the pulpit, and Al-Hasan ibn ‘Ali was by his side. The Prophet, sallallaahu ‘alayhi wa sallam, would look once at the people and once at Al-Hasan, saying, "This son of mine is a Sayyid (master), and perhaps Allah will reconcile between two great groups of Muslims by him." [Al-Bukhaari]

What the Prophet, sallallaahu ‘alayhi wa sallam, foretold did actually happen: After ‘Ali was killed as a martyr, the Muslims swore allegiance to his son Al-Hasan, who afterwards gave up the caliphate to Mu‘aawiyah in order to unite the Muslims behind one ruler, sparing them bloodshed and disunity.

Tuhfat Al-Ahwathi reads:

“When ‘Ali was killed in Ramadhaan that year, Al-Hasan spent days thinking about the situation and considering the disunity of Muslims, some supporting him and some supporting Mu‘aawiyah, and he made up his mind that this situation could not be tolerated. He decided that rectifying the affairs of Muslims, sparing them bloodshed, and saving their lives had priority over his right to power. Therefore, he gave up the caliphate to Mu‘aawiyah on the fifth of Rabee’ Awwal, 41 AH. It was also said that this took place in Rabee’ Al-’Aakhir or on the first of Jumaadaa Al-’Oola. Al-Hasan's reign lasted for almost six months. That year was called the year of Jamaa‘ah (unity). This was what the Prophet, sallallahu ‘alayhi wa sallam, had foretold, that perhaps Allah would reconcile two great groups by Al-Hasan.”

Allah knows best.