Hadith: One day the Prophet (may Allah's peace and blessings be upon him) brought Al-Hasan and took him up the pulpit with him and said: This son of mine is a master and hopefully Allah will reconcile by means of him two groups of Muslims

عن أبي بكرة رضي الله عنه قال: أخرج النبي صلى الله عليه وسلم ذات يوم الحسن، فصعد به على المنبر، فقال: «ابني هذا سيِّد، ولعلَّ اللهَ أن يُصلحَ به بين فئتين من المسلمين».
[صحيح] - [رواه البخاري]

Abu Bakrah (may Allah be pleased with him) reported: 'One day the Prophet (may Allah's peace and blessings be upon him) brought Al-Hasan and took him up the pulpit with him and said: "This son of mine is a master and hopefully Allah will reconcile by means of him two groups of Muslims."
[Authentic hadith] - [Narrated by Bukhari]

Explanation

One day the Prophet (may Allah's peace and blessings be upon him) took Al-Hasan, when he was a little boy, to the mosque and got him up the pulpit. He told people that Al-Hasan, his grandson, was a master, i.e. of a noble origin since he belongs to the noblest house on earth. He further said that Allah, the Almighty, would reconcile by means of Al-Hasan two conflicting groups of the Muslims. The dispute would thus end and Muslims would unite. These words of the Prophet (may Allah's peace and blessings be upon him) certainly attest to his prophet-hood. He said that this noble master Al-Hasan ibn ‘Ali (may Allah be pleased with him) would end inter-Muslim conflicts and get Muslims together in unity. This actually took place when Al-Hasan relinquished the office of the caliphate in favor of Mu‘āwiyah, thereby preventing a bloodshed and creating unity in the Year of Jamā‘ah (40 or 41 AH).