On the night before Ashura when Imaam Hussain was passing by Umm-e-Farwa's tent he heard Qasim say to his mother, "Mother, tomorrow Uncle Abbas, Ali Akber and I will defend Imaam Hussain. Mother, if I get killed please do not weep for me." Umm-e-Farwa replied, "My son although I dearly love you, I shall not weep for you. Nothing will make me prouder than to see my son give his life for Islam". When Imaam Hussain heard this conversation he prayed to Allah to bless Hazrat Qasim and Umm-e-Farwa.
On Ashura day after Aun and Muhammad had fallen in the battlefield, Hazrat Qasim came to Imaam Hussain and begged for permission to go and fight. Twice Imaam Hussain refused saying "Qasim you are young and your mother's only son". Hazrat Qasim was very disappointed. He went to his mother. When she saw her son so disappointed she remembered that just before he died, Imaam Hassan had written a letter, given it to Umm-e-Farwa saying ,"If ever you find Qasim in difficulty, give him this letter." She gave Hazrat Qasim the letter. In the letter it was written, "My son Qasim, a day will come when my brother Hussain will be facing an enemy army of tens of thousands. That will be the day when Islam will need to be saved by sacrifice. You must represent me on that day."
Hazrat Qasim read the letter. He smiled. He rushed to Imaam and gave him the letter. After reading the letter Imaam Hussain said, "O my brother's son, how can I stop you from doing what your father wanted you to do. Bismillah, go. Allah be with you".
Imaam Hussain tied Imaam Hassan's turban on the head of Hazrat Qasim and helped him mount his horse.As Qasim was riding out, Imaam Hussain said "Inna Lilla-hi Wa Inna Ilai-hi Raji'oon". Hazrat Qasim said "O my uncle - Do not be upset. I do not fear death. Death for Islam will be sweeter for me than honey". Hazrat Qasim came to the battle field. He was a very handsome boy. When the enemy saw him, they began to murmur, "How can we kill someone whose face is shining like moon?"
In a loud and a clear voice Hazrat Qasim introduced himself and cried out a challenge for single combat. He killed several famous warriors who came forward.
Every time he felled an enemy Hazrat Qasim would stand on his stirrups, look behind and wave to Hazrat Abbas, just as a pupil would seek approval of his teacher on any assignment, Hazrat Abbas would proudly wave back. When the enemy saw his bravery and realized that they could not possibly over power him in single combats, they came from behind and one of them hit Hazrat Qasim on the head with a sword.
Hazrat Qasim tried to go on fighting but alas he was so young and he was so very thirsty! He fell from his horse crying out, "YA AMMAHO, ACCEPT MY LAST SALAAMS." Imaam Hussain and Hazrat Abbas rode out. The soldiers tried to stop them, when finally they came to where Hazrat Qasim had fallen they saw a dreadful sight!! Hazrat Qasim's body had been trampled by the horses of the soldiers who had tried to stop Imaam Hussain and Hazrat Abbas. Hazrat Abbas was so angry when he saw this that he began to shake with fury. He took out his sword and wanted to attack the enemy. Imaam Hussain restrained him saying "Abbas, have patience, dear brother. Do not give them an excuse for saying that we attacked them first".
Was Qasim body carried to the camp to his mother? This is not very clear. It is reported, however, that Imaam Hussain took off his abaa, spread it on to the ground, gathered the pieces from the ground as one gathers flowers from a garden!