Husayn ibn Ali (Arabic: الحسين بن علي, romanized: al-Ḥusayn ibn ʿAlī; 11 January 626 – 10 October 680 CE) was a social, political and religious leader in early medieval Arabia. The grandson of the Islamic prophet Muhammad and an Alid (the son of Ali ibn Abi Talib and Muhammad's daughter Fatima), as well as a younger brother of Hasan ibn Ali, Husayn is regarded as the third Imam in Shia Islam after his brother, Hasan, and before his son, Ali al-Sajjad. Husayn is a prominent member of the Ahl al-Bayt and is also considered to be a member of the Ahl al-Kisa and a participant in the event of the mubahala. Muhammad described him and his brother, Hasan, as the leaders of the youth of paradise.
During the caliphate of Ali, Husayn accompanied him in wars. After the assassination of Ali, he obeyed his brother in recognizing the Hasan–Mu'awiya I treaty, despite it being suggested to do otherwise. In the nine-year period between Hasan's abdication in AH 41 (660) and his death in AH 49 or 50 (669 or 670), Hasan and Husayn retreated to Medina, trying to keep aloof from political involvement for or against Mu'awiya I. After the death of Hasan, when Iraqis turned to Husayn, concerning an uprising, Husayn instructed them to wait as long as Mu'awiya was alive due to Hasan's peace treaty with him.