Imam Hasan AS started to impart the knowledge of Rasulullah feeling safe from Mu'awya's threat due to the treaty. Soon people realized the attrcities of Mu'awya towards the Shariat. Imam Hasan's greatness bagan to spread its own word. On the other hand Mu'awiya began to realize his mistake of accepting the truce , as Imam Hasan bacame safe and the Shariah strong both of which he did not like.
It was Mu'awiya's ardent desire to impose his son Yazid (who had been named after his uncle) upon the Muslims by making him the succeeding Khalifa, despite the fact that Yazid was the playboy of the time, with many evil habits including gambling, heavy drinking, and indulgence in the pleasures of the flesh. But the peace agreement would not permit Mu'awiya to appoint Yazid as his successor, (According to the agreement Imam Hasan would immediately become Khalifa upon Mu'awiya's death). Therefore, it was obvious to Mu'awiya that, if Al-Hasan did not outlive him, Mu'awiya could do as he pleased. Thus Mu'awiya planned to kill Imam Al-Hasan in order to pave the way for his son Yazid to be his successor.
Mu'awiya sent one of his agents to contact Imam Al-Hasan's wife Jada who was the daughter of Al-Ash'ath (once a secret agent for Mu'awiya against Imam Ali in the Battle of Siffin). Jada was asked a small favor, i.e., to put a little poison in Al-Hasan's food, and in return Mu'awiya would give her a large sum of money and also make her wife of his son Yazid. It is said the due to some family matters Hasan Imam was cross with her and therefore Mu'awiya took this opportunity to frighten her that Hasan was to devorce her. The greed and fear played its role. She found the offer too attractive to ignore, and foolishly agreed to accept it. A few days later, she mixed poison in honey and gave it to the Imam. As soon as the Imam took the poisoned honey he became seriously ill. This continued to a period extending to a month. Jada then sent to Mu'awiya that she had tried every poison on Hasan but failed to succeed in bringing him death. Mu'awiya sent her powdered diamond (Hira-Kani) that would cut the internal organ of Hasan into pieces. The poisan had played its role. Sensing that his death was imminent, the Imam designated his brother Al-Husain (a.s.) for Imamat. Although Imam Al-Hasan knew he was poisoned, he did not reveal that to anyone but to his brother Al-Husain.
One thing Al-Hasan had wished was to have his burial by the side of his grandfather, Prophet Mohammed (pbuh). Imam Husain made all the arrangements to fulfill that wish but Mu'awiya's governor over Medina did not let that happen and used military force to stop it. Imam Al-Hasan was 47 year old when he died of poisoning. He was buried near Moulatena Fatema in Baqi.
Medina was never the same without Imam Hasan (a.s.). Everyone missed him dearly. People at first did not believe Mu'awiya poisoned Imam Al-Hasan, but soon found out the truth.
When Husain AS was assended the throne of Imamat he was 46 years old. Imam Husain AS carried on with his mission of teaching Islam as before. A large number of people kept coming to see him and to learn from him. This process continued for several years when people began to hear an ugly rumor that Mu'awiya wanted his son, Yazid, to succeed him.