Imam Ali (a) wrote this letter to Salman al-Farsi, before his caliphate.
This world is like serpent - so soft to touch yet so lethal in its bite. Therefore, try to avoid those things of this place which please you or allure you because this world will be with you for a very short time and will be of very little use to you. Do not keep your mind fully engrossed in mundane affairs because you know for certain that you will shortly leave this world.
Be most wary and cautious of this vicious world at times when it allures and pleases you the most because it is an old trick of this world that when a man is most happy with pleasure of owning and possessing it, it suddenly deserts him and when a man is most confident of its protection, help and love, it certainly forsakes him.