Abu Muhammad Ali bin Hussain, known as Zayn al-Abidin (ornament of the pious) and also by the titles of as-Sajjad (the prostrator) and az-Zaki (the pure), was born in Medina on 38/658. Dhahabi (d. 748/1348 ) writes in "Tadhkiratul Huffaz" that Imam Muhammad Bakir narrated, "Whenever my father mentioned any blessing of God, or whenever any worldly trouble was averted, or whenever he reconciled two quarrelling persons, or whenever he finished any prayer - on such occasions he used to offer prostration (sajda), therefore, he was called as Sajjad." As for the title az-Zaki(pure), it is said that he had left to involve into worldly turmoils, and led a pious life in Medina. He would feed the hungry persons at night, from one to three hundred families; and in daytime, he would have a hundred sheep a day killed for meat, which would be distributed to the needy people. Much of his time he spent sitting on an old piece of matting, fasting all day, or eating a little barely bread. D.M. Donaldson writes in "The Shiite Religion" (London, 1933, p. 110) that, "One day, he claimed to get nourishment from merely the smell of food."
It was the year of Zayn al-Abidin's birth that the edifice of Islamic solidarity was rudely shaken by a band of seceders from Ali's army, known as the Kharijis. Zayn al-Abidin was 2 years old during the martyr of Ali bin Abu Talib, and about 22 years and 6 months old during the event of Karbala. In his personal appearance, Zayn al-Abidin is described as much like Ali. He was about the same height, had reddish hair, a white face and neck, and a large chest and stomach.
Shah-i Zanan, al-Sulafa, or Harar, better known as Shahr Banu, the mother of Zayn al-Abidin was the daughter of the last Sassanid emperor Yazdigard (d. 31/652) of Iran. Tradition has it that during the caliphate of Ali bin Abu Talib, his governor at Fars, Hurais bin Jabir had sent two daughters of Yazdigard as captives to Medina, one of whom was married to Imam Hussain, called Shahr Banu, and other to Muhammad bin Abu Bakr. The popular legend relating the presence of Shahr Banu at Karbala is quite untrue, and cannot be ascertained from any known source. She however is reported to have died soon after the birth of her son, Zayn al-Abidin. In memory of his mother, Zayn al-Abidin used to utter these words: "I am the son of the two chosen stocks (ibn al-khairatain). The Prophet was my grandfather, and my mother was the daughter of Yazdigard."
Imam Hussain had expressly appointed Zayn al-Abidin as his successor. The most commonly reported tradition in this connection, according to "Bihar al-Anwar" (11th vol., p. 7) by Muhammad Bakir Majlisi, is that Hussain, before leaving for Kufa, entrusted Umm Salmah bint Abu Umayyah Suhail, the widow of the Muhammad, who outlived all the wives and died in 63/682; with his will and letters, enjoining her to hand them over to the eldest of his male offspring in case he himself did not return. Zayn al-Abidin was the only son who came back and so he was given his father's will and became his nominee. According to "Usul al-Kafi" (1st vol., p. 149), "Verily, al-Hussain bin Ali, leaving for Iraq, entrusted the book and his will (istawda al-kitab wal wassiya) to Umm Salmah, and when his son Ali bin al-Hussain returned to Medina, she handed these over to him." Another tradition relates (vide Kulaini's "Usul al-Kafi", 1st vol., p. 353) that Hussain nominated Zayn al-Abidin as his successor and the next Imam of the house of the Prophet just before he went out to meet the Umayyad forces for the last encounter at Karbala.
With the exception of few incidents, the life of Zayn al-Abidin is shrouded in the political intrigues. He had however witnessed the rule of the six Umayyads caliphs, viz. Muawiya bin Abu Sufian (40-60/661-680), Yazid I (60-64/680-683), Muawiya II, Marwan bin Hakam (64-65/683-685), Abdul Malik (65-86/685-705) and Walid (86-96/705-715). He had also seen the reigns of Abdullah bin Zubayr and Mukhtar Thaqafi, but kept himself out of the vortex of politics.