Sheikh Khâlid Husayn
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