Hadith No: 1
Ibn Abi al-Hadid - Sharh Nahj al-Balagha
Volume: (16) - Page Number: (11)
[The text is lengthy, so only the relevant excerpt is provided here]
- .... Abu al-Hasan al-Mada'ini said: And his death was in the year forty-nine, and his illness lasted forty days, and he was forty-seven years old. Muawiya had poison brought to him through the hand of Jada bint al-Ashtath ibn Qays, the wife of al-Hasan, and said to her: "If you kill him with poison, you will receive one hundred thousand, and I will marry you to my son Yazid." When he died, Muawiya fulfilled his promise to her regarding the money but did not marry her to Yazid, saying: "I fear you might do to my son what you did to the son of the Messenger of Allah (peace be upon him and his holy progeny)."....

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Reference: Sharh Nahj al-Balaaghah Book 16

Hadith No: 2
Ibn Abi al-Hadid - Sharh Nahj al-Balaghah
Volume: (16) - Page Number: (29)
[The text is lengthy, so only the relevant excerpt is provided here]
- .... Al-Mufid said: And he died a martyr, poisoned, which Muawiya secretly administered to him and to Sa'd ibn Abi Waqqas when he wanted to appoint his son Yazid as his successor after him. They both died from it within a few days of each other, and it was Hasan (peace be upon him) who orchestrated this with his wife, Ju'dah bint al-Asht'ath ibn Qays, in exchange for money that Muawiya offered her ....

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Reference: Sharh Nahj al-Balaaghah Book 16