Mahdi is from the progeny of Imam Husain عليه السلام
Kitāb al-Ghayba|Volume 1|Book 1|Chapter 30
158- وفي رواية أخرى أنه قبض ليلة إحدى وعشرين وضرب ليلة تسع عشرة. وهي الأظهر.
H 158 - In another traditional report, it is mentioned that Imam عليه السلام was wounded on the eve of the 19th and he passed away on the 21st. Obviously the second report is more accurate.