As per the source

Imam Hasan al-Askari

Al-Askari

Birth: 4 Rabi al-Thani, 232 AH (Medina)
Martyrdom / Passing: 8 Rabi al-Awwal, 260 AH (Samarra; by poisoning, per Shia narration)
Relation to Ahl al-Bayt: Son of Imam Ali al-Hadi; the eleventh Imam; father of Imam al-Mahdi

He spent most of his life under close confinement in Samarra (known as al-'Askar). His son Muhammad - whose identity was concealed by the Shia community for protection - is believed to be the awaited Imam al-Mahdi.

Teachings

His careful, discreet preparation for his son's concealment amid an acute political threat.

Related occasions

Shahadat of Imam al-Askari