Based on the 4 sources of Quran, hadis, ijma [consensus], and qiyas [analogy], Islamic jurisprudence [fiqh] classifies all human actions on a scale of relative religious value, ranging from obligatory acts [farz/fard] to those which are forbidden [haram].
The categories are: obligatory [farz], forbidden [haram], commendable [mandub], abominable [makruh], and permissible [mubah]. Mandub, in turn, is sub-categorized into: sunna mu'akkada [ommision of lact leading to rebuke, but not punishment, e'g. azan]; sunna nafila [a practice which the Prophet sometimes carried out, but not on every occasion]; and sunna al-mustahab [a desirable, though not obligatory, practice in imitation of the Prophet, such as his way of walking]. See al-Azimi, On Schacht's Origins, pp 34-5.
In the legal context the term 'sunna' has a range of specific meanings, being a sub-class that falls under the category of commendable [mandub] acts.
The Muslim desire to imitate the beloved Prophet Mustapha (Allah bless him and give him peace) in all spheres of life necessarily meant that any belief, idea, or practice that came into use after the Prophet's lifetime became problematic. The term for the latter is bida [literally 'innovation'], that for which 'there is no precedent in the time of the Prophet'. As the opposite of Sunna, which is 'old' [qadim], it also has the meaning of 'new' [muhdath or hadath]. [J. Robson, 'Bida', EL2, vol 1, p. 1199]
The author Goldziher [Muslim Studies, vol 2, pg 34] adds that 'in general bida is something arbitrary that springs from individual insight and the admissibility of which is not documented in the sources of religious life'. Consequently the term bida has the connotation of 'reprehensible'. Thus the hadis: "May he who introduces new things into this town [Medina] be cursed by Allah, his angels and all men"..
Tracing the history of the use of the term bida in Islamic scholarship, Khalid Masud believes that until the 16th century the term was used in a 'general and vague' way by traditionalists [muhaddithun] and theologians [mutakalilimun], and was relatively little used in a legal sense by the fuqara. Moreover, it was applied strictly to religious beliefs and practices [ibada], rather than to social customs in general. [Muhammad Khalid Masud, 'Trends in the Interpretation of Islamic law as Reflected in the fatawa literature of Deoband school, M. A. thesis, Institute of Islamic studies (Montreal: McGill University, 1969, pg 17)]
In the 19th century al-Shafi [767-820] had made a broad distinction between a 'good' and 'bad' bida. As Robson says,
a distinction came to be made between a bida which was 'good' [hsana] or praiseworthy [mahmuda] and one which was 'bad' [sayyi'a] or blameworthy [madhmuma]. Al-Shafi laid down the principle that any innovation which runs contrary to the Quran, the sunna, idjma, or athar [a tradition traced only to a Companion or a Follower] is an erring innovation, whereas any good things introduced which does not run counter to any of these sources is praiseworthy.
In due course Islamic jurisprudence classified bida into 5 classes, ranging from the obligatory to the prohibited, on the basis of the general principle described above. In the early 19th century, however, the Wahhabis in Arabia 'claimed to go back to the early sunna of the Prophet in contradiction to legal schools'. Goldziher is strongly critical of this development:
Modern Wahhabism follows the pattern of ealier times in striving to brand as bida not only anything contrary to the spirit of the sunna, but also everything that cannot be proved to be in it. It is known that ultra-conservative opposed every novelty, the use of coffee and tobacco, as well as printing, coming under this heading.
In the 19th century British India the 'ulema' did not at all define bida in the same way. The Deobandi scholars Rashid Ahmad Gangohi [1828-1905] and Ashraf Ali Thanawi [1863-1943] regarded 'every new thing...in conflict with sunnah [as] bid'ah. According to them the domain of bid'ah [was] only ibadat or strictly speaking, religious practices'. Strongly condemning such practices such as the urs of saints, the giyarhawin of Shaikh Abd al-Qadir Jilani (may Allah sanctify his soul), and other customs favored by the Ahl-e Sunnat, the Deobandis and the Ahl-e Hadis accused the Ahle Sunnat wa Jamaat of being 'bidatis'. For the Ahle Sunnat wa Jamaat, by contrast, there was 'good' bida and 'bad'. This point will be more fully elaborated in the context of the Azaan [call to prayer] debate.