"The death of Imam Jafar Sadik in 148/765 marked the beginning of the decentralization of Shi'ite religious authority. The Ithna Asharites or the Twelvers, the Shi'ite sect supported Musa Kazim as their next Imam after Imam Jafar Sadik. Musa Kazim was born in 128/745 on the road between Mecca and Medina. His mother was a Berbar slave, called Hamida. Throughout the whole of his life, Musa was faced with hostility from the Abbasids. The cause of his arrest and murder is said to have been the result of the plotting of caliph Harun ar-Rashid's vizir, Yahya bin Khalid. Thus, Musa was arrested in 177/793 in Mecca, then he was sent to Baghdad, where he was imprisoned and killed by poisoning in 183/799.
Kashi writes in Ikhtiyar Marifat al-Rijal (Tehran, 1964, p 459) that, "At the time when Musa Kazim was imprisoned, an amount of thirty thousand dinars for khums had been deposited with his two agents in Kufa. One of these agents was Hayyan al-Sarraj. The two agents spent this money in buying houses and trading, and made considerable profit. When Musa Kazim died and the news reached them, they denied his death and spread the story that the Imam had not died, because he was the promised Mahdi and disappeared. This group became known as the Waqifiyya." A.A. Sachedina writes in The Just Ruler (New York, 1988, p. 54) that, "It emerges from this story that the idea of the occultation of al-Kazim may possibly have been invented by those agents who wanted to benefit from the material wealth that could have been claimed by the succeeding Imam."
The followers of Musa Kazim became known as the Musawiyya. He was followed by Ali ar-Rida, who was born in Medina in 148/765. Soon after the death of caliph Harun ar-Rashid, the Abbasid empire was split between his sons, Amin and Mamun. Amin was defeated in a civil war and Mamun's army under the Iranian General, Tahir, occupied Baghdad. Caliph Mamun summoned Ali ar-Rida from Medina, and appointed him his heir-apparent in 201/816. Ali ar-Rida died in 203/818 at Tus. He was succeeded by Muhammad at-Taqi, who was born in 195/810. He came to Baghdad shortly after his father's death. Caliph Mamun warmly received him and gave his daughter in marriage to Muhammad at-Taqi. Caliph Mamun died in 218/833, and was succeeded by his brother, Mu'tasim. Muhammad at-Taqi was summoned back to Baghdad in 220/835, where he died in the same year. He was followed by Ali al-Hadi, the tenth Imam of the Twelvers. He was born in 212/827 in Medina. During the reign of the Abbasid caliph Mutawakkil, both the Twelvers and Mutazilis came under an intense persecution. In 233/848, Ali al-Hadi was summoned to Samarra, where he lived for twenty years under the observation of the Abbasid spies. He died in Samarra in 254/868. The eleventh Imam of the Twelvers was Hasan al-Askari, who was born in 232/846 in Medina. His period lasted for six years and died in 260/874.
Perhaps no aspect of the history of the Twelvers is as confused as the stories relating to their twelfth Imam and this is not surprising as this is the point in their history where the events related become of a miraculous, extra-ordinary nature and the non-believer may be unwilling to go along with the facts as related by the Twelvers. The following story is the one presented in their later traditions:-
The mother of twelfth Imam was a Byzantine slave-girl, named Narjis Khatoon (or Saqil or Sawsan or Rayhana). The tenth Imam, Ali al-Hadi, bought her for his son, Hasan al-Askari. The twelfth Imam, who was named Mahdi, is supposed to have born in 255/868 in Samarra. Some later sources vary by as much as five years from this date. Ibn Babuya (d. 381/991) writes in Kamal ad-Din (1:222) that, "The 11th Imam's reported complaint that none of his forefathers had been as much doubted by the faithful as he was." Hasan al-Askari died in 260/874, but it however appears that none of the notables knew of the birth of al-Mahdi, and so they went to Jafar, the brother of Hasan al-Askari, assuming that he was now the Imam. Jafar was asked about the son of Hasan al-Askari, but he remained unshakable in his assertion that his brother had no progeny. For this reason, Jafar has been vilified in later sources as a liar (khadhhab). Muhammad Manzoor writes in Iranian Revolution (Karachi, 1988, p. 105) that, "Traditions relating to the birth, disappearance and the concealment of the twelfth Imam are given in several chapters of Usul al-Kafi, such as, from pages 202 to 207, and 333 to 342. A persual of them will convince that the whole "case" is fabricated and even that has not been done skillfully and well, and the version of Imam Hasan Askari's brother and other family members appears to be correct and worthy of belief." Nawbakhti writes in his Fiaraq (p. 79) that, "Hasan al-Askari died and no offspring (khalaf) or vestige (athar) was seen after him. As no apparent child for him was known, his inheritance was divided between his brother Jafar and mother." Thus, the idea of the Imam's occultation was invented by a necessity, because the line of the Alids vanished, it was necessary to maintain the spiritual force