Hasan al-Basree 110 AH (may Allah have mercy on him) said,

إن القلوب تموت وتحيا
فإذا هي ماتت فاحملوها على الفرائض
فإذا حييت فأدبوها في التطوع

“Indeed, hearts dies and lives.

If the hearts are dead then (give it life by) carrying out the obligations, and if the hearts are alive then refine it by doing voluntary actions.”

Narrated by Ahmad in az-Zuhd 233