Passing Thoughts ...

“Ward off passing thoughts, for if you do not, they will become ideas. Ward off ideas, for if you do not, they will become desires.Fight the desires, for if you do not, they will become resolve and determination. And if you do not ward them off, they will become actions. If you do not resist them with its opposite, they will become habits.And it will be difficult for you to get rid of them." [al-Fawâid by Ibn al-Qayyim al-Jawziyyah]

From here.