The difference in terminology among scholars is one of the issues that is usually a difference in wording. That is because terminology results from a scholar choosing a particular word to refer to something; hence when differences occur, the difference is in wording but not in meaning.
With regard to the issue of differentiating between the words “Sunnah” and “hadeeth”, we may say that these two words may mean the same thing in some contexts, and may mean different things in other contexts.
Firstly: places where they mean the same thing
Whatever is narrated from or about the Prophet (blessings and peace of Allah be upon him) of words, actions or approval may be called “hadeeth” or it may be called “Sunnah.”
Shaykh ‘Abdullah al-Judayyi‘ said:
The basic meaning of the word “Sunnah” is fundamentally the same as the definition given by hadeeth scholars, as mentioned above for the word “hadeeth”, when mentioned in general terms without anything to describe what is being spoken of. That excludes the reports that speak of the physical description of the Prophet (blessings and peace of Allah be upon him); but this exclusion is only when talking about the Sunnah in the context of it being one of the sources of sharee‘ah. In that context, the reports that speak of his description are not part of the Sunnah; rather the Sunnah is only based on his words, deeds and approval. End quote.
Tahreer ‘Uloom al-Hadeeth
One of the names of the saved group, the group that will continue to adhere to the commands of Allah, is “Ahl al-Hadeeth.” They are also called “Ahl as-Sunnah.”
The books that deal with the transmission of reports from the Prophet (blessings and peace of Allah be upon him) and the Sahaabah, and the words of the righteous early generations, are called “Kutub al-Hadeeth”; they are also called “Kutub as-Sunnah.”
Secondly: places where they mean different things:
The term Sunnah is applied to the Prophet's (may the peace and blessings of Allah be on him) guidance which is well-established in a general sense in all of his affairs, meaning his path, his methodology and his way. In this context, the scholars do not usually use the term “hadeeth.”
Al-‘Allaamah as-Sayyid Sulaymaan an-Nadwi (may Allah have mercy on him) said:
“Hadeeth” refers to every incident that is attributed to the Prophet (peace be upon him), even if he only did it once in his life, and even if it was narrated from him by only one person. With regard to the word “Sunnah,” in reality it is used of his actions – I mean how the Messenger (peace be upon him) did things – that has been transmitted to us via mutawaatir actions, in the sense that the Prophet (peace be upon him) did it, then the Sahaabah did it after him, then the Taabi‘een did it after them, and so on. It does not necessarily mean that there is a narration describing that action; rather this refers to the way in which a particular act was performed and was passed down through the generations by means of tawaatur (i.e., passed down by so many to so many that it is inconceivable that they could have all agreed upon a lie). This is what is called “Sunnah,” and this is mentioned alongside the Book (the Qur’an) in the hadeeth in which the Prophet (peace be upon him) said: “I am leaving among you two things and you will never go astray so long as you adhere to them: The Book of Allah, may He be exalted, and the Sunnah of His Messenger.” This is what it is not permissible for any Muslim, no matter who he is, to forsake or go against, otherwise he has no share in Islam. End quote.
Majallat al-Manaar, 30/673
The scholars use the word “Sunnah” to describe adhering to Islam in the manner prescribed, without adding to it or introducing innovations into the religion; they do not call that “hadeeth.” For example, ‘Abd ar-Rahmaan ibn Mahdi al-Mashhoor said: Sufyaan ath-Thawri is an imam (leading scholar) in hadeeth, but he is not an imam in Sunnah; al-Awzaa‘i is an imam in Sunnah but he is not an imam in hadeeth. Maalik ibn Anas is an imam in both.
Tareekh Dimashq by Ibn ‘Asaakir, 35/183
Al-Haafiz Abu ‘Amr ibn as-Salaah (may Allah have mercy on him) was asked:
Some of them said of Imam Maalik (may Allah be pleased with him) that he combined knowledge of the Sunnah and of hadeeth; what is the difference between the Sunnah and hadeeth?
He (may Allah be pleased with him) said:
In this context, Sunnah is the opposite of bid‘ah (innovation). A person may be ascholar of hadeeth yet also be an innovator. Maalik (may Allah be pleased with him) combined both Sunnahs; he was a scholar of the Sunnah, i.e., hadeeth, and also believed in the Sunnah, i.e., his way was that of Ahl al-haqq (the people of truth), with no bid ‘ah (innovation). And Allah knows best. End quote.
Fataawa ibn as-Salaah, 1/139-140
The fuqaha’ use the word “Sunnah” when explaining the ruling on doing a specific action as being mustahabb (liked or encouraged); they do not use the word “hadeeth” in this context.
When the scholars speak of reports and whether they are saheeh (sound) or da‘eef (weak), they only use the word “hadeeth”; they do not use the word “Sunnah.” So they say “This is a da‘eef (weak) hadeeth”; they do not say “This is a da‘eef Sunnah”, because of them the Sunnah is that which is proven in the hadeeths; therefore they sometimes say “This hadeeth is contrary to qiyaas (analogy), the Sunnah and ijmaa‘ (scholarly consensus).”
And Allah knows best.