The ninth of Muharram is called Tasu’a. Imam al-Sadiq (عليه السلام) is reported to have said,
On the ninth of Muharram, Imam al-Husayn, along with his companions, was besieged in Karbala’ when the Syrian horsemen surrounded him leaving for him but a narrow area. The son of Marjanah (i.e. ‘Ubaydullah ibn Ziyad) and ‘Umar ibn Sa’d were delighted due to the great numbers of the horsemen who participated in that encounter against Imam al-Husayn (عليه السلام). Thus, the enemy became sure that no supporter would join the army of Imam al-Husayn (عليه السلام) and the people of Iraq, in particular, would not back him.
Imam al-Sadiq(عليه السلام) then added,
May I sacrifice my father for the weak and the stranger (i.e. Imam al-Husayn).