Cassim's Dagger is a dagger that belonged to Cassim before he gave it to his son through his wife, before leaving his family so he could find a treasure that could make their lives better then before. The hand mark on the dagger's hilt is the same mark that is seen on Cassim's cloak.
How Cassim acquired the dagger is unknown but it was presumably a part of the original cargo that carried the legendary Hand of Midas. The dagger eventually found its way into Cassim's possession which likely proved to him that the legendary Hand of Midas did exist. When Aladdin was still an infant, his father Cassim went off into the world to find this gold treasure for his family so they could finally stop living as lowly street rats, but before leaving, he left his wife and son his dagger to remember him by. Years later upon discovering the ship of Midas, Cassim returned to Agrabah only to discover that his wife had perished and his son was never found. But unbeknownst to him, his son still lived wandering the streets of Agrabah as a street rat while holding onto his father's dagger.
The dagger served as Aladdin's only connection to his father and kept safely hidden in case he ever lost it or had it taken away from him. On the day of his and Jasmine's wedding, he went to fetch the dagger from its hiding place so a part of his father would be with him as he was about to take a big step in life.
When Aladdin learned that his father is alive, he brought the dagger with him as he followed the Forty Thieves to their hideout where he learned that his father is the King of Thieves. Cassim believed Aladdin's claims of being his son when he laid eyes on his old dagger. With Sa'Luk wanting to challenge Aladdin to a dual that would allow the Forty Thieves to spare his life should he win, Cassim gave the dagger back to his son so he would have a weapon to fight with.
The next night, Aladdin had freed his father from the palace dungeon and escaped to the outskirts of the city. Cassim was ready to flee Agrabah and never look back and wished for Aladdin to join him, seeing that his life in Agrabah was over the moment Aladdin rescued him. However, Aladdin refused to leave Jasmine behind, realizing he didn’t want to repeat his father’s mistakes like how Cassim had left his wife and child all those years ago. Aladdin gave the dagger back to him by throwing it to his feet, symbolizing his disownment to his father. However after Iago warns Aladdin that Cassim had been taken by Sa'Luk and taken to the Vanishing Isle, Aladdin let's go of his frustration and goes to save his father who in turn realizes that his son truly is his most prized treasure, thus restoring their familial ties, while Cassim keeps the dagger with him as a memory of his son as he travels the world with Iago.