Al-Tawba (The Repentance) - Chapter 9: Verse 92 (partial)
Grief is anguish over something that has elapsed, or regret over something that cannot be had.
It consists of three levels:
The first level is grief of common people; which is grief over negligence in carrying out one's devotion, embroiling oneself in alienation and over days that are lost.
The second level is the grief of people of willpower; it is grief over the suspension of time because of spiritual dispersion, over being side-tracked from contemplation and over being consoled from grief. For the people of distinction, the station of grief is in no way their own.
However, the third level of grief relates to those who are saddened by obstacles rather than concepts, by the opposition against the aims and by objections against stipulations.
"Stations of the Wayfarers" - Abdullah Al-Ansari, p. 64