|The Fourth Imam|
Imam Sajjad (Ali ibn Husayn entitled Zayn al-'abidin and Sajjad) was the son of the third Imam and his wife, the queen among women, the daughter of Yazdigird the king of Iran. He was the only son of Imam Husayn to survive, for his other three brothers Ali Akbar, aged twenty-five, five year old Ja'far and Ali Asghar (or 'Abdallah) who was a suckling baby were martyred during the event of Karbala. The Imam had also accompanied his father on the journey that terminated fatally in Karbala, but because of severe illness and the inability to carry arms or participate in fighting he was prevented from taking part in the holy war and being martyred. So he was sent with the womenfolk to Damascus. After spending a period in imprisonment he was sent with honor to Medina because Yazid wanted to conciliate public opinion. But for a second time, by the order of the Umayyad caliph, 'Abd al-Malik, he was chained and sent from Medina to Damascus and then again returned to Medina.
The fourth Imam, upon returning to Medina, retired from public life completely, closed the door of his house to strangers and spent his time in worship. He was in contact only with the elite among the Shi'ites such as Abu Hamzah Thumali, Abu Khalid Kabuli and the like. The elite disseminated among the Shi'ah and the religious sciences they learned from the Imam. In this way Shi'ism spread considerably and showed its effects during the imamate of the fifth Imam. Among the works of the fourth Imam is a book called Sahifah sajjadiyah. It consists of fifty-seven prayers concerning the most sublime Divine sciences and is known as "The Psalm of the Household of the Prophet."
The fourth Imam died (according to some Shi'ite traditions poisoned by Walid ibn 'Abd al-Malik through the instigation of the Umayyad caliph Hisham) in 95/712 after thirty-five years of imamate.