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.
The dagger serves as Aladdin's only connection to his father and was 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 goes 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 learns that his father is alive, he brings the dagger with him as he followed the Forty Thieves to their hideout where he learns that his father is the King of Thieves. Cassim believes Aladdin's claims of being his son when he lays eyes on his old dagger. With Sa'Luk wanting to challenge Aladdin to a duel that would allow the Forty Thieves to spare his life should he win, Cassim gives the dagger back to his son so he would have a weapon with which to fight.
The next night, Aladdin frees his father from the palace dungeon and escapes 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 refuses 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 gives the dagger back to him by throwing it to his feet, symbolizing his disownment to his father.
The dagger is likely a khanjar.