Imam Hassan Askari (peace be upon him) passed away on the morning of Friday, the 8th of Rabi’ al-Awwal in the year 260 AH, during the Fajr prayer. Ibn Babawayh and others have reported that the Abbasid Caliph, al-Mu’tamid, poisoned the Imam, leading to his martyrdom.
Abu Sahl recounts: “I was in the presence of Imam Askari (peace be upon him) when the Imam instructed his servant to boil some water. Once the water was ready, the mother of Imam Mahdi (may Allah hasten his reappearance) brought it to the Imam. As she was about to hand the cup to the Imam for him to drink, his blessed hand trembled and the cup struck his front teeth. The Imam placed the cup on the ground and said to his servant: ‘Go into this room, and you will see a child in prostration; bring him to me.'”
Abu Sahl says the servant went into the room and found a child who was prostrating with his finger raised towards the sky. The servant greeted the child, and the child responded. The servant then informed the Imam that Imam Askari (peace be upon him) had called for him. At that moment, the child’s mother entered, took his hand, and led him to his father.
Abu Sahl continues: “When the child reached the presence of Imam Hassan Askari (peace be upon him) and greeted him, I looked at his face and saw that it had a radiant glow. His hair was curly and tangled, and his teeth were slightly apart. As soon as Imam Askari (peace be upon him) saw his son, he began to weep and said: ‘O master of your family, give me some water, for I am about to go to my Lord.’ The child took the cup, placed it to his father’s lips, and quenched his thirst. Then Imam Askari (peace be upon him) said: ‘My son, rejoice, for you are the Mahdi, the Proof of God on Earth, and you are my son and successor. I am your father, and you are Muhammad ibn al-Hasan ibn Ali ibn Muhammad ibn Ali ibn Musa ibn Ja’far ibn Muhammad ibn Ali ibn al-Husayn ibn Ali ibn Abi Talib (peace be upon them). Your father is the Messenger of God (peace be upon him), and you are the final Imam of the pure line of Imams. Your name is the same as the name of the Messenger of God. This is the covenant passed down to me from my father and the pure ancestors…’
It was at that moment that Imam Hassan Askari (peace be upon him) passed away.
(Adapted from the book Al-Ghaybah, by Sheikh Tusi)
Thus, by the divine will of Allah, the sacred existence of Imam al-Mahdi (may Allah hasten his reappearance) took on the mantle of Imamate after his noble father. In his early childhood, much like Prophet Yahya (John) and Prophet Isa (Jesus) (peace be upon them) who were appointed to prophethood in their youth, Imam Mahdi (peace be upon him) was entrusted with the divine leadership of the Ummah. Even though Imam Mahdi (peace be upon him) entered into occultation after the martyrdom of his father in order to protect himself from the enemies, his father had introduced him to the Shi’a before his divine occultation, affirming the Imamate of his son and emphasizing it as a matter of divine decree.