The Sunnah is a source.
In the terminology of fiqh (Islamic jurisprudence), sunnah denotes whatever though not obligatory, is "firmly established (thabata) as called for (matlub)" in Islam "on the basis of a legal proof (dall shar`)
 Ibn Qutaybah (213-276 AH) distinguished between:Sunnah "brought by Gabriel";sunnah from "Muhammad's own ra'y and is binding, but subject to revision";"non-binding sunnah", which Muslims are not subject to "penalty for failure to follow"

