Abi Jameeleh quoted on the authority of Imam Sadiq (a.s) that Ameer al-Momineen (a.s) wrote a letter to advise one of his companions
“I will advise myself and you to fear God, the God whose disobedience is not permissible, and there is not any one else on whom we can place any hopes, or can make us self-sufficient. In fact whoever feared God became honorable, strong, satisfied and their intelligence was raised above that of the people of the world. Their bodies are with the people of this world but their heart and intellect are attentive to the Hereafter. Their spiritual illumination of the heart has put out any love for this world that their eyes might have experienced. They recognize the forbidden things in this world and abstain from the doubtful things. By God they have even abandoned the purely allowed things in this world except for a little they are obliged to, and that being a piece of cloth to cover them. Even then they use the most rugged clothes, and the worst food they can get, and they do not place any hopes on them either. They only place their hopes on the Creator of the two worlds. Thus they will work hard, and strain their bodies until their bones can be seen and their eyes are filled with tears. God will reward them with physical and mental strength, and increase their reward in the Hereafter. Reject the lowest form of existence that is this world since love for this world will make you deaf, dumb, blind, and humiliated. Try to compensate during the rest of your life, since those who lived before you were ruined because they insisted on their high hopes, and their procrastinations, and then suddenly the time of death approached by God's order while they were negligent. Then they were placed in the coffin and dispatched to their dark and narrow graves and left their family and children. You leave the world with an attentive heart, having cut off hopes in the world and everyone in it, and go towards God with an unbreakable determination. May God help you and us to obey Him, and succeed in pleasing Him.”