The 4th Imam after Imam Hussain(a.s.) was his son Imam Ali Zain-al-Abideen(a.s.). His mother was Bibi Shahar Bano who was a princess from Persia, the daughter of the King Yazd Gard II. She was brought as a prisoner of war during the caliphate period of mam Ali(a.s.) during 31 A.H. and Imam Ali(a.s.) got her freed and married her to Imam Hussain(a.s.). Imam Zain-al-Abideen was born from this wedlock. She, however, died within 10 days of the birth of Imam Sajjad(a.s.).
His title Zain-al-Abideen was granted to him by the Holy Prophet(pbuh&hf) himself who mentioned that on the day of judgement a call for Zain-al-Abideen will be made and my son Ali bin al-Hussain(a.s.) will responde to the call saying “Labbaik”. His other Title, Syed-us-Sajideen, was given because of his devotion to prayers. He would pray for long durations specially during the nights and would pray a lot of prayers of gratitude – Namaz-e-Shukrana.
…The calm and peaceful life of the Imam was not to be tolerated by the Ummayad’s cruel regime. They realized that the Imam was succeeding in his mission of spreading the message of his father Hussain(as) The Syrian monarch Walid Ibn Abdul Malik had him poisoned. He died in Medina on the 25th of Muharram 95 Hijri. His eldest son Muhammad Ibn Ali al Baqir arranged the burial and laid him to rest in the grave yard of Jannatul Baqii beside his uncle Imam Hasan.(AS)