Ibn Al Qayyim (rahimahullaah) wrote:
“Repel the thought, for if you don’t, it becomes an idea.
So repel the idea, for if you don’t it will become a desire.
So fight against that (desire), for if you don’t, it will become a determination and a passion.
And if you don’t repel that, it will become an action.
And if you don’t replace it with its opposite, it will become a constant habit.
So at that point, it will be difficult for you to change it.”