Imam Sufyaan once wrote to one of his Muslim brothers
May Allaah, with His Mercy, protect both me and you from the Hellfire.
I advise both you and myself to fear Allaah.
And I warn you not to turn to ignorance after you have learned, not to turn to destruction after you have seen and recognized the truth, and not to leave the (Straight) Path after it has been made clear to you.
Do not become deceived by worldly people, and do not become unduly impressed by the way they strive for and greedily gather worldly things, for the terror (that will envelop them on the Day of Resurrection) is indeed SEVERE.
The danger (of the Day of Resurrection) is grave indeed, but what is more, is that it is near at hand.
So occupy yourself exclusively with the Hereafter, and empty your heart of all other thoughts; once you have done that, work hard! Do not waste time! And flee from the world and its temptations! Travel to the Hereafter (with your worship) before you are taken there… I have indeed given you the same advice that I gave myself.
And understand that success comes from Allaah (subhaanahu wa ta’aala). The key to gaining His help is supplication, prayer, feeling remorse for past negligence, and complete submission to Him.
Your days and nights are numbered, so use the time you have left wisely, and do not be negligent in fulfilling the rights of your Lord.
I ask Allaah, Who has blessed us to know Him, not to entrust us and you to our ownselves; and I ask Allaah to be our Guardian and Helper, just as He is the Guardian and Helper of His beloved, righteous slaves.
Beware of doing that which will spoil your deeds, and know that it is showing off that spoils a person’s deeds; and if it is not showing off, then it is conceitedness – for you to imagine that you are better than one of your Muslim brothers, when in reality he performs more good deeds than you do; or perhaps he performs his deeds with a purer intention than you do.
And even if you are not conceited, beware of developing a love for praise.
Be ever so careful not to come to love other people’s praise of your good deeds, or the respect they feel for you and bestow upon you because of your good deeds.
And beware of desiring that others should help you in your personal affairs just because they have become impressed with your good deeds.
You surely claim (as does everyone else), after all, that you are doing good deeds only for the sake of Allaah (subhaanahu wa ta'ala); well, then turn that claim into a reality.
If you want to be less desirous of this world – its things, wealth, and pleasures – and more desirous of the Hereafter — Paradise and its bliss – then remember death frequently.
And know that you have long term hopes for this world – which one should not have — if you fear Allaah only a little or if you recklessly perpetrate sins.
And one will be sufficiently regretful and wretched on the Day of Resurrection if he has knowledge but does not apply it.
* Transcribed from: The Biography of Sufyaan Ath-Thauree, rahimahullaah, page 181-182