Supererogatory Prayer

Hearts have [the capacity to] incline [to worship] and to retract, so when they incline then perform the supererogatory, and when they retract then observe the obligatory.

رسولُ اللَّهِ صلى اللَّه عليه وآله : إنّ لِلقُلوبِ إقبالاً وإدباراً ، فإذا أقبَلَت فتَنَفَّلوا ، وإذا أدبَرَت فعلَيكُم بالفَريضَةِ. الكافي : 3 / 454 / 16 .

 The Prophet (SAWA) said, 'Hearts have [the capacity to] incline [to worship] and to retract, so when they incline then perform the supererogatory, and when they retract then observe the obligatory.' al-Kafi, v. 3, p. 454, no. 16