summary:
Imam Abu Muhammad Hassan al-Askari(A.S.) was poisoned by the caliph, Mu'tamad, and, thus, suffered severely and was martyred on 8th of the month of Rabi' Awwal in the year 260 (A.H.). At the time of his death, he was twenty-eight years old. He was buried in the house in which his father was buried in Samarra'. He left behind his son - Living Holy Awaited Saviour Imam Muhammad al-Mahdi (A.S.) the one who is awaited to bring about the heavenly rule of truth in the whole world.
His name is al-Hasan, Abu Muhammad. Being a resident of "Asgar a suburb of Samarra", he is titled al-Askari. His father was Imam "Ali An-Naqi (A.S.) and his mother was Salil Khatun, an ideal woman in piety, worship, chastity and generosity. He was born in Medina on the tenth of Rabi" al-Akhir, 232 A.H.
He lived under the care of his respected father unto the age of 11. When his father had to leave for Samarra" he was to accompany him and thus share the hardships of the journey with the family. At Samarra", he passed his time with his father either in imprisonment or in partial freedom. He had, however, the chance to benefit from his father’s teaching and instruction.
Imam Abu Muhammad Hassan al-Askari(A.S.) was poisoned by the caliph, Mu'tamad, and, thus, suffered severely and was martyred on 8th of the month of Rabi' Awwal in the year 260 (A.H.). At the time of his death, he was twenty-eight years old. He was buried in the house in which his father was buried in Samarra'. He left behind his son - Living Holy Awaited Saviour Imam Muhammad al-Mahdi (A.S.) the one who is awaited to bring about the heavenly rule of truth in the whole world.
source:http://english.tebyan.net/newindex.aspx?pid=64483&Keyword=+Imam+Hassan+Askari