imam hussein peace be upon him said: some people worship god because of their desire for paradise: this is the businessman's worship. Some people worship god because of their fear of hell: this is the slave's worship. And some people worship god because of gratitude: this is the free person's worship, and it is the best one.
Don't use "this" after a comma ( , ). You can put "which" after a comma, or you can add a conjunction like "and", but here I thought that the best choice was to use a colon ( : ) instead of a comma. If you say "his thanks," it sounds like God is thanking the worshipers--here, "gratitude" is a better choice.