It is that which, in excess of what is needed, brings virtue to a person.
Farāʾiḍ (obligatory acts) are those matters which Allah has commanded His servants to perform or to abstain from. Whoever does not fulfill these becomes sinful. These are of two types: farḍ al-ʿayn (individual obligation) is an obligation that is not lifted from a person by someone else performing it. Farḍ al-kifāya (communal obligation) is an obligation that is lifted from an individual if someone else performs it. (Futūḥāt, TI, 165)
One of the signs of a true murīd (aspirant, disciple) is that he fulfills the sunnah (the Prophetic practice) and the farḍ (obligatory acts) as defined in our terminology. The sunnah is to renounce the world, while the farḍ is to persist in the dhikr (remembrance of God) of Allah... (Anwār, I, 138)