Name : Hasan ibn Ali - the 11th Holy Imam
Title : al-Askari
Agnomen : Abu Muhammad
Father : Imam Ali Naqi-al-Hadi(a.s.) - the 10th Holy Imam
Mother : Bibi Hadithah ( or Susan )
Birth : At Madina, on Monday, 10th Rabi-us-Sani 232 AH (845 AD)
Martyred : In Samarrah at the age of 28 on Friday, 8th Rabi-ul-Awwal 260 AH (872 AD)
Cause of Death/Burial: Poisoned by al-Mu`tamid ,the Abbasid ruler and buried at Samarrah in Iraq
Imam Hasan al-Askari(a.s.) spent twenty-two years of his life under the patronage of his father, Imam Ali Naqi al-Hadi(a.s.) after whose martyrdom he became his divinely commissioned Imam . Imam Hasan al-Askari(a.s.), the son of the Tenth Imam , was born in 232 AH and according to some Shi`ite sources was poisoned and killed in 260 AH through the instigation of the Abbasid caliph al-Mu`tamid . The Eleventh Imam (a.s.) gained the Imamate, after the death of his noble father, through Divine Command and through the decree of the previous Imams. During the seven years of his Imamate, due to untold restrictions placed upon him by the caliphate, he lived in hiding and dissimulation (taqiyyah) . He did not have any social contact with even the common people among the Shi`ite population. Only the elite of the Shi`ah were able to see him. Even so, he spent most of his time in prison. There was extreme repression at that time because the Shi`ite population had reached a considerable level in both numbers and power. Everyone knew that the Shi`ah believed in the Imamate and the identity of the Shi`ite Imams was also known. Therefore, the caliphate kept the Imams under its close supervision more than ever before. It tried through every possible means and through secret plans to remove and destroy them. Also , the caliphate had come to know that the elite among the Shi`ah believed that the Eleventh Imam , according to traditions cited by him as well as his , forefathers , would have a son who was the promised Mahdi . The coming of the Mahdi had been foretold in authenticated hadith of the Prophet (pbuh&hf) in both Sunni and Shi`ite sources. For this reason the Eleventh Imam (a.s.), more than other Imams, was kept under close watch by the caliphate.
The caliph of the time had decided definitely to put an end to the Imamate in Shi`ism through every possible means and to close the door to the Imamate once and for all. Therefore, as soon as the news of the illness of Imam Hasan Askari(a.s.) reached al-Mu`tamid , he sent a physician and a few of his trusted agents and judges to the house of the Imam(a.s.) to be with him and observe his condition and the situation within his house at all times .After the death of the Imam(a.s.), they had the house investigated and all his female slaves examined by the midwife . For two years the secret ageins of the caliph searched for the successor of the Imam (a.s.) until they lost all hope. The Eleventh Imam (a.s.) was buried in his house in Samarrah next to his noble father.
Birth & Childhood
When he was born his father said that it was an order from the prophet s.a.w. that he should be named Hasan. It was so well known that the last Imams father would be called Hasan that people had previously looked for the 12th Imam amongst the sons of Imam Hasan Al-Mujtaba (a.s.)
His mother was Sulail (or Hudaitha) who was a slave of the 10th Imam whom he had freed and subsequently married. The 10th Imam said of her that she was one who was free from all vice and pollution and that she was one of the righteous ones. After the 11th Imam's martyrdom he left all the "orders" of Imamate with her (his mother).
Once when Imam Hasan Al-Askari(a.s.) was a young child he fell into a well near their house. His father Imam Ali An-Naqi(a.s.) was praying and didn't even notice the commotion around him of the ladies crying. When he finished praying and asked what had happened, he was told. He went towards the well. The water rose and all saw the 11th Imam (a.s.) sitting peacefully on the water and playing.
Bahlool once saw the young Imam crying in the street where all the other children were busy playing with their toys. He asked whether Imam (a.s.) was crying because he had no toys to play with. Imam (a.s.) replied: "We have not been created to play!" Bahlool asked where he had learnt this from and the Imam said: "Have you not read the Qur'an where it says - "Do you think we have created you in vain and that you will not be returned to us". When Bahlool heard this he asked Imam (a.s.) for some advice. Imam (a.s.) talked about death, barzakh and qiyama and cried so much, he fainted. When he came to, Bahlool asked him why he was as fearful as he was not even blight yet. Imam (a.s.) said: "I have watched my mother light a fire; she uses the little twigs to get the big pieces of wood to burn; I fear that on qiyama, Allah (swt) too will use little ones to light the big ones".
Title: Al-Askery
The 10th and 11th Imamayn were both called Al-Askery as they lived in an area of Samarra called Askery. Another reason that the 11th Imam was called Askery is said to be that once Mutawakkil wanted to show the Imam the strength of his army of 90,000 men and so he asked his men to fill their sacks with sand and pile all the sacks up. He then showed Imam this mountain of sacks. Imam asked Mutawakkil, to look through his (Imam's) two fingers, where he saw a an army of armed Angels. Imam told him that they were at his disposal but he would not use them. (An army is called Askery).
Life & Works
Ahmed Bin Ishaq relates that when he heard of the death of the 10th Imam he went to Samarra and asked for the whereabouts of the 11th Imam. He was told that Imam had been imprisoned by Mo'taz Billah. After bribing the guards he was able to visit Imam one night. He described the prison as a tunnel under the Khalifa's home where there was no space to stretch ones legs or room to be able to stand. Ahmed says he cried when he saw Imams condition. Food was only one glass of water and a piece of dry bread a day. He was imprisoned because all the rulers knew of the justice that was promised to come with the coming of the 12th Imam and they wished to prevent anyone being born of the 11th Imam. Whilst in prison he told his companions to accumulate all the masails of Fiqh and he completed the masails on the chapters that were found missing. He introduced the institution of "Taqleed" advising people to follow those who were learned, "Muttaqi", and just, as he was in prison and it was extremely difficult for people to meet him. Khums was collected through his representative and masails too asked to him, Abu-Ja'fer Uthman bin Saeed, who was later to become the representative of the 12th Imam.
There was once a severe drought in Samarra and a Christian priest came who whenever he raised his hands caused rain to fall from the heavens. The Muslims faith started wavering and the Khalifa Mo'taz Billah got worried for if they left Islam he would have none to rule over. He went to the 11th Imam saying that Imam's grandfather's religion was in trouble. Imam asked him to call all the people outside Samarra with the Christian priest. He asked the Christian priest to pray for rain. When he raised his hands to pray it started to rain. Imam asked for whatever was in these hands to be taken away and then asked him to pray for rain again. There was no rain this time. Imam showed that what the priest had in this hand was the bone of a Prophet which whenever placed under the open sky caused it to rain.
Imam then prayed for rain and it rained. Mo'taz could not re imprison Imam for some time for when the people saw the miracle they all wanted to visit Imam and asked where they might find him. Imam pointed to the Khalifa and asked them to ask him. Mo'taz told them that Imam was staying at the house where the 10th Imam used to reside.
Bibi Nargis was there and it was in that year that the 12th Imam was born. Imam was allowed out of prison for 13 months after which he was rearrested.
Martyrdom
In the six years of his Imamate several attempts were made on his life. On the 1st of Rabi-ul Awwal 260 a.h. Mo'tamad arranged for Imam (a.s.) to be poisoned in prison and then sent him home. Imam (a.s.) was in great pain and difficulty for 8 days. On the 8th of Rabi-ul Awwal he asked his son, the 12th Imam, to bring him some water and then leave him. He died that day and was given ghusl & kafan by his son, Imam Mahdi(a.s.).
Burial
the funeral prayers were led by the 12th Imam and it was then that a lot of Shia's saw him for the first time. A large number of people attended the funeral and he is buried in Samarra.
source : http://shiastudies.net