Hasan ibn Ali, second imam of the Shi'ites, is poisoned and killed
Muawiyah wished to pass the caliphate to his own son Yazid ibn Muawiyah, and saw Hasan ibn Ali as an obstacle to his plans.
And thus Muawiyah plotted to kill Hasan ibn Ali. He secretly contacted Hasan ibn Ali's wife Ja'da bint al-Ash'ath ibn Qays, and instigated her to poison her husband. Ja'da did as Muawiyah suggested, giving her husband poison mixed with honey. Madelung (pp. 331–333) notes other traditions suggesting that Hasan ibn Ali had been poisoned by another wife, the daughter of Suhayl ibn Amr, or perhaps by a servant. Madelung also cites the early historians (Baladhuri, Waqidi, etc.) who recounted these traditions. Madelung, who is more accepting of Shi'a traditions than most Western academic historians, believes that Hasan ibn Ali was poisoned and that the famous early Islamic historian al-Tabari suppressed the tale out of concern for the faith of the common people. (Madelung pp. 331–332)
Shi'ahs believe that Ja'da was promised gold and marriage to Yazid. Seduced by the promise of money and power, she poisoned her husband, and then hastened to the court of Muawiyah in Damascus to receive her reward. Muawiyah reneged on his promises and married her to another man.
Hasan ibn Ali died in Medina on Safar 28, 50 AH. He is buried at the famous Jannatul Baqee‘ cemetery across from the Masjid al-Nabawi, the Mosque of the Prophet.