How did the classical scholars define Sunnah?

In a Nutshell:  The classical scholars differed on defining the term Sunnah. Scholars of hadith (muhaditheen) consider Sunnah as everything narrated concerning the life of the Prophet (saw) as well as opinions of the companions and their successors. Scholars of usul al-fiqh (usuliyeen) include only what the Prophet did, said or approved. Scholars of jurisprudence (fuqaha) generally use the term to refer to what is recommended (mustahab or mandub). Theologians (mutakallimeen) didn't define the term as they were more concerned with orthodox theological teachings and innovations (bid'ah) in the creed (i'tiqad). Background Sunnah linguistically refers to several possible meanings; a way issued by earlier peoples and followed by later ones, circularity of face, image of someone or something, the nature or attribute and the story of a certain nation. Classical Scholars differ on how to define the term "sunnah" technically, both within and outside their discipline. Scholars of Hadith (known as ahl al-hadith or muhaditheen) compile the prophet's narrations, including things such as morals, history, sayings, actions, approvals, attributes and others, whether these narrations carry a legal ruling or not. All of these elements are included when they speak of sunnah. On the other hand, scholars of usul al-fiqh (principles of jurisprudence) search for the legislator Prophet (saw), more concerned with hadiths from which we can deduce rulings.  Jurists search for the intended rulings, such as mandatory, prohibited, recommended, disliked or permissible of such narrations. Theologians search for the theological narrations support the orthodox beliefs and refute innovations (bid'ah). Differences thus primarily arise based on their discipline - but that is not to say there are not inter-disciplinary differences. (Abd al-Ghani, Hujiyat as-Sunnah, pp. 57-58) Sunnah According to Hadith Scholars (Muhaditheen) Hadith scholars are more concerned with compiling and narrating the hadiths, so consider the sunnah as everything narrated concerning the life of the Prophet (saw) before or after the initiation of revelation, so it equals the hadith. This includes his sayings, acts, approvals, attributes and even a wave by hand, regardless of their rule, abrogation meaning. It also includes what the companions narrated about the Prophet (al-Marfu') and their successors (Maqtu'). For example, Imam Bayhaqi compiled his Book (Asunan al-Kubra); whilst his book is called (sunnah), he included statements of the companions and their successors. (Asfahani, Hulyat al-Awliya', Vol. 6, p. 332, Sam'uni, Tawjih an-Nadhar, Vol. 3, p. 2, Noah, Shifa' as-Sudur, pp. 8-13, Siba'i, as-Sunnah wa Makanatuha, 57) They cited this hadith to support their opinion, the Prophet (saw) said: فَعَلَيْكُمْ بِسُنَّتِي وَسُنَّةِ الْخُلَفَاءِ الرَّاشِدِينَ الْمَهْدِيينَ، عَضُّوا عَلَيْهَا بِالنَّوَاجِذِ "You must keep to my Sunnah and to the Sunnah of the Khulafa ar-Rashideen (the rightly guided caliphs), those who guide to the right way. Cling to it stubbornly (literally: with your molar teeth)." (Tirmidhi) This group of scholars classify Sunnah into general five categories:  Sunnah Qawliyyah: every word the Prophet (saw) said during his life. For example, the Prophet (saw) said: إِنَّمَا الأَعْمَالُ بِالنِّيَّاتِ "The reward of deeds depends upon the intentions." (Sahih al-Bukhari 1) Sunnah Fi'liyyah: The Prophet's actions, attitudes, educational application of Allah's rules and marriage relationship. For example, it was narrated the Prophet (saw) said: صَلُّوا كَمَا رَأَيْتُمُونِي أُصَلِي "Perform your prayers in the way you saw praying." (Sahih al-Bukhari 6008) So when the prophet (saw) prayed, the companions imitated him and they narrated his preformation to us. So Abdullah ibn Umar narrated: أَلاَ أُخْبِرُكُمْ بِصَلاَةِ، رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ فَقَامَ فَرَفَعَ يَدَيْهِ أَوَّلَ مَرَّةٍ ثُمَّ لَمْ يُعِدْ "Shall I not tell you about the prayer of the Messenger of Allah (saw)? He stood and raised his hands the first time and then he did not do that again." (Sunan an-Nasa'i 1026) Sunnan Taqririyyah: Every action or statement issued by someone during the attendance of the Prophet, or he knew about it, but he did not condemn it. For example, it was narrated on the authority of Jabir (ra): كُنَّا نَعْزِلُ عَلَى عَهْدِ النَّبِيِّ صلى الله عليه وسلم وَالْقُرْآنُ يَنْزِلُ‏.‏ "We used to practice coitus interrupt us during the lifetime of Allah's Messenger (saw) while the Qur'an was being Revealed." (Sahih al-Bukhari 5209) So approval is considered legal because the Prophet (saw) would not keep silent about prohibited matters. Sifatuhu al-Khuluqiyah or Khalqiyah (moral or physical attributes) refers to the moral and physical attributes of the Prophet (saw). For example, it was narrated: ِكَانَ رَسُولُ اللهِ صَلَى اللهُ عَلَيْهِ وسَلَمَ أَشَدُ حَيَاءً مِنَ العَذْرَاءَ فِي خِدْرِهَا، فَإِذَا رَأَى شَيْئًا يَكْرَهَهُ عَرِفْنَاهُ فِي وَجْهِه "Messenger of Allah (saw) was even shier than a virgin behind her veil. When he saw something which he disliked, we could perceive it on his face." (Sahih al-Bukhari and Muslim) When Aisha (ra) was asked about the character of the prophet (saw), he replied: َكَانَ خُلُقُه القُرْآن. His character was the Qur'an. (Al-Adab al-Mufrad 308) The marfu' narrations (not attributed directly to the Prophet) and the maqttu' narrations (of the tabi'een) are considered part of the sunnah because the companions (ra) must have heard it from the prophet (saw) and taught it to the successors (ra). Sunnah According to Scholars of Usul al-Fiqh (Usuliyun) Scholars of Usul al-Fiqh (principles of jurisprudence) are primarily concerned with the Sunnah of the Prophet (saw) as the second source of legislation after the Qur'an. So they only included the first three categories: The Prophet's sayings, actions and approvals. The theoretical jurist al-Ijji (d. 756 A.H.) says: ٍمَا صَدَرَ عَنْ سَيِّدِنَا مُحَمَدٌ صَلَى اللهُ عَلَيهِ وسَلَمَ –غَيْرَ القُرْآنِ- مِنْ فِعْلٍ أوْ قَوْلٍ أوْ تًقْرِير. "What has been issued by the Prophet Muhammad (saw), outside the Qur'an, such as action, statement or approval." (al-Ijji, Sharh Mukhtasar ibn Rajab, Vol. 2, p. 22) The Sunnah could also refer to the Ijma' of the companion as evidence of legislation, not the marfu' hadiths, such as the consensus of the companion on the collection of the Qur'an. (Shattibi, al-Muafaqat Vol. 4, p. 6) Sunnah According to Jurists (Fuqaha) Scholars of jurisprudence, generally, use the term Sunnah to refer to what is recommended (Mustahab or Mandub). So they say for the divorce that fulfils the prescribed conditions (Talaqu-Sunnah) divorced according to the Sunnah, and the divorce that does not fulfil the conditions is called (Talaqu-Bid'ah) innovated divorce that is not recommended. So Sunnah here has nothing to do with the narrations but has everything to do with the ruling of a certain deed. (Shawkani, Irshad al-Fuhul, p. 31) Jurjani said: و(السُنَةُ) فِي الشَرِيعَةِ هِيَّ الطَرِيقَةُ المَسْلُوكَة فِي الدِينِ مِنْ غَيْرِ افْتِرَاضِ وُجُوبٍ، فَالسُنة مَا وَاظَبَ النَبِي صَلَى الله عَلَيهِ وَسَلَمَ عَلَيْهَا مَعَ التَرْكِ أَحْيَانًا. "(Sunnah) in the Shari'ah is the conductive way in the Religion apart from mandatory rulings. Sunnah is what the Prophet (saw) continuously used to perform and he sometimes left performing it." (Jurjani, at-Ta'rifat, p. 161) Abu al-Baqa' al-Kufi said: ٍوشَرْعًا اسْمٌ لِلطَرِيقَةِ المُرْضِيَةِ المَسْلُوكَةِ فِي الدِينِ مِنْ غَيْرِ اَفْتِرَاضٍ ولَا وُجُوب. "Technically, it refers to the acceptable conductive way in the Religion apart from mandatory rulings." (Abu al-Baqa' al-Kufi, Mu'jam fi al-Mustalahat, p, 783) Jurists disagree on the meaning of Sunnah when it is used in an unrestricted manner. Most of the earlier Hanafi jurists and the jurists of the other madhahib (schools) adhere it refers to only the Sunnah of the Prophet (saw), but later Hanafi scholars, such as Bazwidi (d. 730 A.H.), al-Karkhi (d. 340 A.H.) and others, argue it includes the Sunnah of the Prophet (saw) and his companions. (Nasafi, Kashf al-Asrar, Vol. 1, p. 296, Rahawi, Hashiyah Sharh ibn Malk, Vol. 2, p. 587)   Sunnah According to the Theologians Theologians (ullama al-aqidah/mutakallimeen) were more concerned with theological teachings and innovations (bid'ah) in the creed (i'tiqad). They refer to orthodox beliefs as the Sunnah and opposing beliefs as bid'ah. The Hanbali jurist and theologian Ibn Rajab (d. 795 A.H.) said:   َوالسُنَةُ هِيّ الطَرِيقُ المَسْلُوكِ، فَيَشْمَلَ ذلك التَمَسُكِ بِمَا كَانَ عَلَيْهِ هُوَ وخُلَفَاؤُهُ الرَاشِدُونَ مِنْ الاعْتِقَادَاتِ والأَعْمَالِ والأَقْوَالِ وَهَذِهِ هِيَّ السُنَة كَامِلَةً، ولِهَذَا كَانَ السَلَف قَدِيمًا لا يُطْلِقُونَ اسْم السُنَةَ إِلَا عَلَى مَا يَشْمَلُ ذَلِكَ كُلِهِ.  "Sunnah is the conductive way, it includes adherence to the conduct of (the Prophet) and his rightly guided Caliphs (ra) regarding creed, actions and sayings. This is the complete Sunnah. Therefore, the earlier forefathers used the term Sunnah to refer only to what comprises all of these matters." (Ibn Rajab, Jami' al-'Ulum, p. 295) Shawkani said: ِوتُطْلَقُ السُنَةُ عَلَى مَا يُقَابِلُ البِدْعَةَ كَقَوْلِهِم فُلَانٌ مِنْ أَهْلِ السُنَة. "Sunnah refers to the counter of bid'ah as they say 'so and so is an adherent of Sunnah.'" (Shawkani, Irshad al-Fuhul, p. 31) Which means he does not believe, do, or say a bid'ah. (Abd al-Ghani, Hujiyat as-Sunnah) Conclusion Classical scholars disagree on the meaning of the Sunnah depending on their discipline. Scholars of hadith were more concerned with narrations of hadiths, so they consider the Sunnah as everything narrated concerning the life of the Prophet (saw) before or after the initiation of revelation, as well as the statements of the companions and their successors. Scholars of Usul al-Fiqh (the principles of jurisprudence) are only concerned with the sunnah as the second source of legislation after the Qur'an, so they only consider what the prophet did, said or approved as sunnah. Scholars of jurisprudence, generally, use the term Sunnah to refer to what is recommended (mustahab or mandub). Theologians focused on orthodox theological teachings (sunnah) and innovations (bid'ah) in the creed (i'tiqad) and did not define sunnah. References abd al-Ghani, Hujiyat as-Sunnah Shawkani, Irshad al-Fuhul Asfahani, Hulyat al-Awliya' Sam'uni, Tawjih an-Nadhar Noah, Shifa' as-Sudur Siba'i, as-Sunnah wa Makanatuha al-Ijji, Sharh Mukhtasar ibn Rajab Shattibi, al-Muafaqat Jurjani, at-Ta'rifat Abu al-Baqa' al-Kufi, Mu'jam fi al-Mustalahat Rahawi, Hashiyah Sharh ibn Malk Nasafi, Kashf al-Asrar Ibn Rajab, Jami' al-'Ulum