Imam al-Hasan al-Askari (a.s.) suffered the great loss of his father, and it was the heaviest calamity he had ever faced in his life.
The Abbasid tyrant al-Mu’tamid assassinated Imam al-Hadi (a.s.) by adding poison to his food, when he saw all people talk prevalently about the virtues and vast knowledge and the high position of the imam. He was very angry and was filled with envy against Imam al-Hadi (a.s.). Then, he committed this terrible crime against him.
After being poisoned, Imam al-Hadi (a.s.) suffered terrible pains and retired to bed. The Shi’a and high statesmen visited him. Some poets visited him like Abu Hashim al-Ja’fari who composed a sorrowful poem on him. Abu Du’amah visited him and when he wanted to leave, the imam said to him,
‘O Abu Du’amah, you have a right on me. Do I tell you a tradition that shall delight you?
Abu Du’amah said, ‘I am in utmost need of that O son of the messenger of Allah.’
Imam al-Hadi (a.s.) said,
‘My father Muhammad bin Ali told me from his father Ali bin Musa bin Ja’far from his father Ja’far bin Muhammad from his father Muhammad bin Ali from his father Ali bin al-Husayn from his father al-Husayn bin Ali from his father Ali bin Abu Talib that the messenger of Allah (a.s.) said to him, ‘Write down!’ Ali said, ‘What shall I write down?’ The messenger of Allah (a.s.) said, ‘Write down: “In the name of Allah, the Beneficent, the Merciful. Faith is that which hearts acknowledge and deeds prove, and Islam is that which tongues witness and marriage becomes lawful with.’
Abu Du’amah said, ‘O son of the messenger of Allah, I do not know which of them is better; the tradition or the narrators!’
Ali al-Hadi (a.s.) said, ‘It is a book written down by the hand of Ali bin Abu Talib and the dictation of the messenger of Allah (a.s.) that we inherit one from the other.’
Before his death, Imam al-Hadi (a.s.) announced that the imam after him would be his son Abu Muhammad al-Hasan al-Askari (a.s.). He appointed him as the general authority for the Shi’a and entrusted him to prepare his corpse (after death), offer the prayer on him, and bury him. He entrusted him with other affairs too.