When you are with people, smile frequently and maintain a cheerful countenance, but when you are alone, cry frequently and be in a state of grief and sadness (on account of your sins).  For it has been conveyed to us — and Allah Knows best — that on the Day of Resurrection, the most things that a believer will find in his book of good deeds are the grief and the sadness (that he felt in this world on account of this sins).

—Sufyān al-Thawrī [d. 161H/778CE]

(Read on pg 170, Salaahud-Deen ibn ‘Alee ibn ‘Abdul-Maujood, The Biography of Sufyaan Ath-Thauree. Darussalam Publishers. Riyadh:2005.)