The eminent Muslim scholar, Sheikh Yusuf Al-Qaradawi, states: “The word Sunnah is used in many meanings, as detailed in the following lines:
1) In the terminology of Usul al-Fiqh (principles of Islamic jurisprudence), it means all that has been reported of the Prophet (peace and blessings be upon him) in a certain way, whether the reports are sayings, actions or tacit approvals. Here the term refers to a source of Shari`ah or Islamic legislation. It is usually mentioned in association with the Glorious Qur’an. It is said, for example: this is confirmed by the Book and the Sunnah.
Sunnah, sometimes, can mean: a legitimate thing, in contrast with bid`ah (an innovation in religion). It is said, for example: doing a few legitimate things is better than doing many innovations in religion.
2) In the Fiqh terminology, it means one of the five judgments under Muslim law, namely: Fard (obligation), Sunnah (recommendable), Haram (forbidden), Makruh (abominable; undesirable) and Mubah (permissible). So, two other words (mandub, mustahabb) may be used to express the same meaning as Sunnah in this specific sense.
It appears that the questioner wants to inquire about things that are proven by the Prophetic Sunnah or traditions: which among them is considered wajib (obligatory) and which is not wajib?
We cannot answer this question before knowing first what such Sunnah has indicated: saying, action or tacit approval. If it were action or tacit approval, it would not as such be evident of anything but mere lawfulness, as the Prophet (peace and blessings be upon him) would never commit a forbidden thing nor tacitly approve anything unlawful. His actions, if indicating a desire to come closer to Allah, may mean recommend ability.
When it comes to proven sayings of the Prophet, we should first look at the formula and its aspects of significance, be it a command or a prohibition, beset with contexts and presumptions or not.
What is clear to me from studying the judgments and deductions of the Islamic jurists – except the Zahirites – is that Sunnah in this specific context is synonymous with recommend ability, unless there is some addition in the text that
is evident of obligation. As for “do not do…” formulas, they usually denote undesirability unless there is some addition in the text that is evident of absolute forbiddance.
Take for instance the Prophet’s saying: “Say: in the Name of Allah at the start of your meal, eat with your right hand and eat of the dish what is nearer to you.” (Reported by al-Bukhari and Muslim) We find those Prophetic commands indicative of recommend ability, but his saying: “And eat with your right hand” in particular indicates obligation, this sense being backed up by another hadith saying: “Let no one of you ever eat with his left hand, because the devil eats with his left hand and drinks with his left hand.” (Reported by Muslim and at-Tirmidhi, on the authority of `Abdullah ibn `Umar) Attributing the act of eating and drinking with the left hand to the devil is sufficient proof that it is forbidden, and necessitates that eating and drinking should be with the right hand.