On the eve of Ashura, Imam al-Hussayn (peace be upon him) gathered his companions and delivered a lecture informing them that they all would be martyred the next day. Al-Qassem son of Imam al-Hassan (peace be upon both of them) asked his uncle; imam al-Hussayn (peace be upon him), whether he would be martyred on Ashura. Imam asked him what he thought about death. He replied, "For your sake, o uncle, sweeter than honey!" Imam then responded, "Yes, O you who is dear to his uncle, you will be martyred tomorrow after fighting fiercely, and my son Abdullah will also be martyred."
On the day of Ashura, in the battlefield in Karbala, al-Qassem son of Imam al-Hassan (peace be upon both of them), showed his courage in the defense of his beloved uncle Al-Hussayn (peace be upon him). He chanted when he faced the enemy:
“If you do not know me I am the son of Al-Hassan,
The grandson of the Prophet, the chosen one, the confidante,
This is Hussayn similar to the one captivated by the mortgager,
In the midst of the people who may be deprived of rain water”.
Al-Qassem (peace be upon him) began to fight the enemies until he killed a large number of them.
Then his left shoes' lace was cut, he leaned down to fix his shoe as he was not worried about the countless fighters surrounding him, who were all fully armoured. That is how some historians narrated this incident -the son of messenger of Allah (Allah's prayers be upon him and upon his holy Household), refused to walk with naked feet in the battlefield-. This was unmatched bravery, and remarkable prowess.
When Al-Qassem bent to mend his shoe, the villainy spirit which was represented by Azdi began to control him. This is what Hameed bin Muslim meant when he narrated the following: Umar bin Sa'ad bin Nufayl Azdi said, “I desire to attack him”. I said, “Glory be to Allah! Why so? This army which has surrounded him from all sides will surely kill him”. He said, “By Allah! I shall attack him, and I will make his mother to weep over him”. He attacked him while the child was fixing his shoe and before he could turn his face towards him, he dealt a blow upon his head with his sword and slit it. The child fell down on his face upon the ground and called out, “Peace be up on you, O dear uncle! Come to my aid”.
It is to mention that in the Ziyarat of Nahiya Al-Muqaddasa, when Imam Al-Hujjah (may Allah hasten his holy reappearance) mentioned Al-Qassem son of Al-Hassan Al-Mojtaba (peace be upon both of them) saying:
"Peace be upon Al-Qassem, the son of Imam Al-Hassan, mortally wounded and distressed, who has called his uncle Al-Hussayn to his rescue, he rushed to him like an eagle and found his young nephew in convulsion, rubbing his heel upon the sand. Then the Imam exclaimed, "Grievous indeed for your uncle that you call him and he does not answer you or help you and his help is not beneficial for you. Away with the people who have killed you. They will be arraigned by your father and grandfather on the Day of Judgement. Truly, this is a day whereon the killers are many and the helpers are few."
Therefore O Al-Qassem ibn Al-Hassan, may the Lord join us with you on the Day of Resurrection and make you home our resort; and may He curse your murderer, Umar bin Sa'd bin Nufail al-Azdi and send the culprit to hell and punish him with a painful torment."