«حكمه: يثاب المرء على فعله، ولا يعاقب بتركه»
One is rewarded for performing a voluntary act and not punished for omitting it.
Compare the four types:
Fard: binding through decisive proof.
Wajib: binding through authoritative but not decisive proof.
Sunnah: an established normative practice to be maintained.
Nafl: additional recommended conduct without punishment for omission.
Their legal force and the strength of their evidence must not be collapsed into one indistinct category.