Hasan–Muawiya treaty

In 661 CE, after Ali's death, Hasan ibn Ali attained to the caliphate. There was a military conflict between Ahl al-Bayt and Muawiya ibn Abu Sufyan (see Battle of Siffin); and to avoid the agonies of a further civil war, Hasan signed the Hasan–Muawiya treaty with Muawiyah. According to the treaty, Hasan ceded the caliphate to Muawiyah, but were to name no successor during his reign but let the Islamic world choose their successor afterward. Muawiya broke this treaty by appointing his son Yazid as his successor.

Muawiyah, the governor of Levant, who had refused Ali's demands for allegiance, has long been in fight with him. However when Ali was assassinated and people gave allegiance to Hasan, Muawiyah prepared to fight with him. The battle led to inconclusive skirmishes between the armies of Hasan and Muawiyah.

Muawiyah who had already started negotiations with Hasan, sent high-level envoys, while committing himself in a witnessed letter to appoint Hasan his successor and give him whatever he wished. Hasan accepted the offer in principle and sent Amr ibn Salima al-Hamdani al-Arhabl and his own brother-in-law Muhammad ibn al-Ash'ath back to Muawiyah as his negotiators, together with the envoys of the latter. Muawiyah then wrote a letter saying that he was making peace with Hasan on the basis that Hasan would inherit the reign after him. He swore that he would not seek to harm him; and that he would give him 1,000,000 dirhams from the treasury (Bayt al-mal) annually, along with the land tax of Fasa and Darabjird; to which Hasan was to send his own tax agents to collect it. The letter was witnessed by the four envoys and dated in August 661.

When al-Hasan read the letter he commented: "He is trying to appeal to my greed for a matter which, if I desired it, I would not surrender to him." Then he sent Abd Allah ibn al-Harith, whose mother Hind was Muawiyah's sister, to Muawiyah, instructing him: "Go to your uncle and tell him: If you grant safety to the people I shall pledge allegiance to you." After which, Mu'awiyah gave him a blank paper with his seal at the bottom, inviting Hasan to write on it whatever he desired.

...
Wikipedia