He was the son of Imam Husayn ibn ‘Ali (‘a), the third Imam of the Shia world, and his mother was Shahr Bānūwiyah.
He was the son of Imam Husayn ibn ‘Ali (‘a), the third Imam of the Shia world, and his mother was Shahr Bānūwiyah. He is most famously known by his agnomens Zayn al-’Abidin and Sajjad.
Imam Sajjad (‘a) was born in the year 28 Hijri and he spent the period of his youth in the city of Medina. He was alive for 2 years during the rule of his grandfather, Amir al-Mu’minin (‘a), and after that, he witnessed 10 years of the Imamate of his uncle, Imam Ḥasan (‘a); during this time, Imam Ḥasan (‘a) was the caliph of the Muslims for a period of 6 months. After the martyrdom of Imam Hasan (‘a) in the year 50 Hijri, he witnessed 10 years of the Imamate of his father, Imam Husayn (‘a). The last years in the Imamate of Imam Husayn (‘a) coincided with the peak of Mū’āwiya’s political power and were filled with constant altercations; Imam Zayn al-’Abidin (‘a) was witness to all of these events.
In the month of Muharram, in the year 61 Hijri, Imam Zayn al-’Abidin (‘a) was present at the revolt of Karbala and the martyrdom of Imam Husayn (‘a) and his companions which followed. After the tragedy of Karbala, in which the Imamate was transferred to him, the Imam (‘a) and the rest of the captives were first taken to Kūfah and then to Syria. During this journey, he was the guardian and support of the rest of the group during the hard times and difficulties which they constantly encountered. During this journey, Imam Zayn al-’Abidin (‘a) disgraced the government of Yazid through his fiery and emotion-evoking sermons. After leaving Syria, he took up Medina as his place of residence. This continued until he was martyred in the year 94 or 95 Hijri. He was buried in the graveyard of Baqi’, next to the grave of his uncle Imam Ḥasan (‘a).
Caliphs Who Lived Contemporaneously With the 4th Imam (‘a)
Imam ‘Ali ibn al-Husayn (‘a) lived contemporaneously with the following caliphs:
1- Yazid ibn Mū’āwiya (61-64 Hijri)
2- ‘Abdullah ibn Zubayr (61-73 Hijri)
3- Mū’āwiya ibn Yazid (A few months in the year 64 Hijri)
4- Marwān ibn Ḥakam (9 months in the year 65 Hijri)
5-’Abd al-Malik ibn Marwān (65-86 Hijri)
6- Walid ibn ‘Abd al-Malik (86-96 Hijri) 
 Muḥammad ibn Ya’qūb Kulayni, Ūṣūl al-Kāfi, Researched and edited by: ‘Ali Akbar al-Ghaffāri, Tehran, Maktabah al-Sadiq, 1381 Hijri Qamari, vol. 1, p. 467; Shaykh Mufid, al-’Irshād, Qum, Maktabah Baṣirati, p. 253; Faḍl ibn Ḥasan Ṭabarsi, ‘I’lām al-warā bi ‘i’lām al-hudā, Ṭab’at al-Thālitha, Tehran, Dār al-Kutub al-Islāmiyah, p. 256; Ḥasan ibn Muḥammad ibn Ḥasan Qummi, The History of Qum, translated by Ḥasan ibn ‘Ali ibn Ḥasan Qummi, edited by: Sayyid Jalāl al-Din Tehrāni, Tehran, Tūs Publications, 1361 Hijri Qamari, p. 196; ‘Ali ibn ‘Isā ‘Irbili, Kashf al-ghammah fi marifat al-’a’immah, Tabriz, Maktabah Bani Hāshimi, 1381 Hijri Qamari, vol. 2, p. 286. In the books of history and the books regarding the lives of the Imams (a), the name of the mother of the 4th Imam (a) is one of which there are varying accounts. In addition to Shahr Bānūwiyah, there are an additional 12 names, which she has been called as well. These are namely: Shahi Zanān, Jahān Shah, Shahr Nāz, Jahān Bānūwiyah, Khawlah, Salāfah, and so on… For further information on this matter, refer to: Doctor Shahidi, Sayyid Ja’far, The Life of ‘Ali ibn al-Ḥusayn (a), First Print, Tehran, The Office for the Propagation of Islamic Culture, 1365 Hijri Qamari, p. 27-29; Doctor Karimān, Ḥusayn, Ray Bāstān, Second Print, Tehran, The Publications Office of the National University of Iran, vol. 1, p. 403-416.  Shaykh Mufid, al-’Irshād, Qum, Maktabah Baṣirati, p. 253; ‘Allāmah Ṭabarsi, Tāj al-mawālid (part of a collection called Majmū’ah Nafsiyah), Maktabah Baṣirati, p. 112; Shaykh Mufid, Masārr al-Shi’ah (part of the same collection), p. 67; Muḥammad ibn Jarir ibn Rustam Ṭabari, Dalā’il al-imāmah, al-ṭab’at al-thālithah, Qum, Rāḍi Publications, 1363 Hijri Qamari, p. 80; Muḥammad ibn Ya’qūb Kulayni, Ūṣūl al-kāfi, Researched and edited by: ‘Ali Akbar al-Ghaffāri, Tehran, Maktabah al-Sadiq, 1381 Hijri Qamari, vol. 1, p. 466; Sibṭ ibn Jawzi, Tadhkirah al-khawāṣ, Najaf, Manshūrāt al-Maṭba’ah al-Ḥaydaria, 1383 Hijri Qamari, p. 324; Mas’ūdi, Ithbāt al-waṣiyah, al-ṭab’at al-rābiah, Najaf, al-Maṭba’at al-Ḥaydaria, 1373 Hijri Qamari, p. 167; Fattāl Nayshābūri, Rawḍat al-wā’iẓin, Researched and Compiled by: al-Shaykh Ḥusayn al-’Alami, al-ṭab’at al-’ūlā, Beirut, Mu’asasat al-’Alami Lil Maṭbū’āt, 1406 Hijri Qamari, p. 222; Faḍl ibn Ḥasan Ṭabarsi, ‘I’lām al-warā bi ‘i’lām al-hudā, Ṭab’at al-Thālitha, Tehran, Dār al-Kutub al-Islāmiyah, p. 256; Ibn Abi al-Thalaj al-Baghdādi, Tārikh al-’a’immah (part of the Nafsiyah Collection), Qum, Maktabah Baṣirati, p. 9. Some historians believe that Imam ‘Ali ibn al-Ḥusayn’s (a) birthday was in the year 36 or 37 Hijri.  The time period mentioned, signifies the time that they ruled during the Imamate of Imam Sajjād (a). They are not inclusive of their entire period of rule.  ‘Abdullah ibn Zubayr was of the small number of individuals who had not given the oath of allegiance to Yazid. After the death of Mū’āwiya, and a little time before the departure of Imam Ḥusayn (a) to Mecca, he entered the city and became engrossed in political activities. After the martyrdom of Imam Ḥusayn (a), since he had no other rivals in the lands of the Ḥijāz, he gathered some supporters and announced himself as the caliph. Yazid, up to the very end of his life, was unable to defeat him, and he ruled in Mecca until 73 Hijri. ‘Abdullah conquered the Ḥijāz, Iraq, Egypt, and part of the eastern Muslim lands, and the successor of Yazid was only able to rule Syria and some other limited areas. Therefore, from the year 61 until 73 Hijri, the Muslim lands were ruled by 2 separate rulers, who each had their own spheres of power. Later on, ‘Abdullah ibn Zubayr was defeated and killed by the forces of ‘Abd al-Malik (in the year 73 Hijri). After this, all of the Muslim lands were ruled under the government of the Marwanids, and Syria was once again considered as the center of rule.  Taken from the text: Sirah Pishvāyān, Mahdi Pishvāi, Mu’asasah Imam Sadiq (‘a), Qum, 1390 Hijri Shamsi, 23rd Edition, p. 233.