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The books of biography and history do not pinpoint anyone as being the murderer of al-Hasan (may Allah be pleased with him).

It is consensus among scholars that he was poisoned. However, al-Hasan refused to declare who poisoned him.

Imam al-Dhahabî relates in his book Siyar A`lâm al-Nubalâ’ (3/273) that al-Husayn asked his brother al-Hasan: "O brother, tell me who gave you this drink (the poisoned one)?"

Al-Hasan said: “Why. So you can kill him?”

He said: “Yes.”

Al-Hasan said: “I will not tell you anything. If it was the one I expect, then Allah will give him a more severe punishment. Otherwise I will not accuse an innocent, so he will be killed.”

The same is related by Ibn Hajr al-`Asqâlânî in al-Isâbah fî Tamyîz al-Sahâbah (2/66).

And Allah knows best.

Source: Islam Today