On the 7th of Safar in 50AH, a tragic event befell on the holy house of Prophecy (peace be upon them), which grief was similar to that felt by the martyrdom of the Holy Prophet, his daughter and the Commander of Faithful (prayers and Allah's peace be upon them), that was the martyrdom of the fourth infallible, Imam Aba Mohammad al-Hassan al-Mojtaba (peace be upon him).
Mu'awyah scared of the wisdom and high position of Imam al-Hassan (peace be upon him) and in order to eradicate all doubts concerning his ability in keeping the takeover of the nation, he has plot to end the life of the Imam (peace be upon him) by poisoning him, so he asked to the king of the Romans to send him lethal poison, but the king of Romans replied: "It is not acceptable in our religion to help in killing those who don't fight against us." But Mu'awyah sent him a letter again telling him: "This man is the son of the one who appeared in the land of Tehama -meaning the holy Prophet (prayers and Allah's peace be upon him and his holy Household)- who wanted the kingdom of your father, and I want to poison him so people and lands can rest.
Then, the king of Romans accepted to send him the lethal poison that Mu'awyah sent to "Ja'dah daughter of al-Ash'ath Ibn Qays" the traitorous wife of Imam al-Hassan (peace be upon him), whose father participated in killing Imam Ali (peace be upon him) and her brother later on in killing Imam al-Hussayn (peace be upon him), promising her to marry her to his son Yazeed -which he did not- and sent her hundred thousand Dirhams to put the poison in the food of the Imam (peace be upon him), once she did, Imam al-Hassan (peace be upon him) became ill for more than forty days, so he (peace be upon him) said his last commandments to his brother Imam al-Hussayn (peace be upon him).
When death was close to Imam al-Hassan (peace he on him) he summoned Imam al-Hussayn (peace be on him) and said: "My brother, I am leaving you and joining my Lord. I have been given poison to drink and have spewed my liver into a basin. I am aware of the person who poisoned me and from where I have been made a subject to this deceitful action. I will oppose him before God, the Mighty and High. Therefore by the right I have with regard to you, say nothing about that and wait for what God, the Mighty and High, will decide concerning me."
Then he said: "Bury me with my grandfather, the Apostle of Allah (prayers and Allah's peace be upon him and his holy Household). For I am worthier of him and his family (than others). If they prevent you from doing so, then I make you swear by the relation which Allah has made close to you and by the blood relation to the Apostle of Allah (prayers and Allah's peace be upon him and his holy Household) not to shed even your blood into the cupping-glass in (carrying out) my command, till we meet the Apostle of Allah (prayers and Allah's peace be upon him and his holy Household) to oppose the people before him and to tell him about what we have suffered from them."
Imam al-Hassan (peace be upon him) has said: "By God, the messenger of Allah (prayers and Allah's peace be upon him) has entrusted us that this matter is owned by twelve Imam from the sons of Ali and Fatima and that there is none of us except that he is poisoned or killed, then he lifted the basin and reclined (peace be upon him)."
Then Imam al-Hassan (peace be upon him) made his testamentary bequests to his family and his children. He gave Imam al-Hussayn (peace be upon him) his heirlooms and the things which the Commander of the faithful; Imam Ali (peace be upon him) had bequeathed to him when he had made him his successor, had declared him worthy to occupy his position, and had indicated to his Shi'a (his followers) that he was his successor, and set him up as their sign-post after himself. Then Imam al-Hassan (peace be upon him) died on the 7th [or 28th] of Safar in the year 50 A.H.
Imam al-Hussayn (peace be upon him) along with the members of his family and companions carried him on his coffin leading towards the grave of his grandfather, the Apostle of God (prayers and Allah's peace be upon him and his holy Household), The escorting processions headed for the Holy Shrine of the Prophet, that they might let Imam al-Hassan renew his covenant with his grandfather and that they might bury him beside his grandfather. When the Umayyads came to know that, they gathered together and armed themselves. Selfishness and the enmity toward the Hashimites moved them to make an opposition and a riot as to burring Imam al-Hassan alongside his grandfather. They refused to let Imam al-Hassan to be buried beside his grandfather. They did not content with that but they threw the coffin with arrows to the extent that seventy arrows were drawn from it. The two parties intended to attack each other. When Imam al-Hussayn (peace be upon him) saw that, he hurried to the Hashimites and said to them: “O Banu Hashim, (fear) Allah and beware of Him! Do not lose the will of my brother. Take him to (the cemetery of) al-Baqi‘. He had adjured me before Allah that if I was prevented from burying him with his grandfather, I should not dispute with anyone as to him and should bury him in the cemetery of al-Baqi’ with his mother.”
Then he turned to the Umayyads and said to them: "By Allah, if there had been no injunction to me from Imam al-Hassan (peace be upon him) to prevent bloodshed and that I should not even pour blood into a cupping-glass in (carrying out) his command, you would have known how the swords of Allah would have taken their toll from you. You have broken the agreement which was made between you and us. You have ignored the conditions which we made with you for ourselves..."
Then they went on with (the body of) Imam al-Hassan (peace be upon him) and they buried him in (the cemetery of) al-Baqi' beside his grandmother, Fatima bint Asad (peace be upon her), and this is how he lived, died and buried oppressed.
And among the narrations recommending weeping over his martyrdom and visiting him is what Ibn Abbas narrated on the authority of the Prophet (Allah's prayers be upon him and upon his holy Household) that he said: “…until he [Imam al-Hassan (peace be upon him)] is killed with poison unjustly. At that moment, the angels and the seven skies cry for his death, and everything weeps for him, even birds in the sky and whales in the depths of water. So whoever weeps over him, his eyes will not be blinded on the day when the eyes will be blind. And whoever grieves for him, his heart will not grieve on the day hearts will grieve. And whoever visits him in his Baqi' will have his foot firm on the [Sirat] on the day when feet will slip.”