Imam As-Sajjad(A.S.) answered the man who asked him for a definition of ascetics:

Asceticism is of ten grades the highest of which the lowest grade of piety. The highest grade of piety is the lowest grade of conviction. The highest grade of conviction is the lowest grade of satisfaction. Asceticism is (summarized) in Allah's saying:

لِّكَيْلَا تَأْسَوْا۟ عَلَىٰ مَا فَاتَكُمْ وَلَا تَفْرَحُوا۟ بِمَآ ءَاتَىٰكُمْ ۗ وَٱللَّهُ لَا يُحِبُّ كُلَّ مُخْتَالٍۢ فَخُورٍ

"... so that you would not grieve over what you have lost nor become too happy about what Allah has granted to you."

(Holy Qur'an 57:23)