By the third Hijri century, the specialists in the field of 'aqeedah (dogma, creed and faith) used the term "sunnah" to refer to the foundations of the faith, the well-established obligatory deeds, the matters of creed and the definitive rulings of Islaam. This terminology became popular as more and more sects appeared. 
Some scholars would use the term "sunnah" to refer to the established articles of faith and to distinguish them from the beliefs of the newly-formed heretical groups. 
Imaam Ibn Rajab al-Hanbali رحمه الله noted, "Many of the later scholars [meaning after the first couple of generations] used the word 'sunnah' to specifically refer to what is related to creed, because that forms the foundation of the religion and the one who contradicts it is in a very dangerous situation." 
Some used the word "sunnah" in a very comprehensive sense to basically include everything that forms the essential parts of the faith - this is acceptable because everything that forms part of the faith was either believed in, practiced or preached by the Prophet صلى الله عليه وسلم himself. For example, sunnah is defined by some as, "The guidance upon which the Prophet صلى الله عليه وسلم and his Companions رضي الله عنهم were, [encompassing all aspects of] knowledge, belief, statement and deeds." 
In this definition, the practices and beliefs of the Companions رضي الله عنهم are considered part of the sunnah because they followed the same methodology as that taught by the Prophet صلى الله عليه وسلم - as opposed to some members of later generations who devised their own methodologies contrary to that of the Prophet صلى الله عليه وسلم.
For the specialists in the field of 'aqeedah, sunnah is also used as the opposite of heresy [بدعة] (bid'ah). 
In this respect, one says, "So and so is upon the sunnah (or following the sunnah)." This means that with respect to his beliefs, overall methodology and behaviour, he is following the way established by the Prophet صلى الله عليه وسلم and passed on to his Companions رضي الله عنهم and who followed in their path. Again, it is important to note that in this sense, the word "sunnah" also encompasses that way of life and belief that the Companions رضي الله عنهم followed as a continuation of what they learned directly from the Prophet صلى الله عليه وسلم. If a person refuses to follow the way of the Companions رضي الله عنهم in their understanding and practice of the religion, they are, in essence, turning away from the sunnah itself.
On the other hand, one says, "So and so is upon an innovation," if he has some beliefs or methodology that contradicts with the pure teachings of Islaam. Those who follow they way of the Prophet صلى الله عليه وسلم are known as Ahlus Sunnah [أهل السنة] or the people of the sunnah. Those who follow any of the myriad of differing ways are known as Ahlul Bid'ah [أهل البدعة] or the people of heresy. Hence, sunnah means the correct belief and understanding of the religion. 
Imaam Abul Qaasim al-Asbahaani رحمه الله said, "So and so is upon the sunnah or from the people of the sunnah means that he is in accord with what has been revealed and also what has been passed on from [from the Companions رضي الله عنهم and others] with respect to his actions and belief. This is so because the sunnah cannot possibly [be present] when one is in contradiction to Allaah سبحانه وتعالى and contradiction to His Messenger (may Allaah exalt his mention and grant him peace)." 
It is in this sense of the word that ahaadeeth such as the following are to be understood: The Prophet صلى الله عليه وسلم said, "Whoever turns away from my sunnah [my way] is not from me." 
The Prophet صلى الله عليه وسلم also said, "Certainly, the one who will live among you will see lots of difference. So stick to my sunnah and the sunnah of the right-principled and rightly-guided successors. Bite onto that with your molar teeth. And avoid newly-introduced matters. Verily, every heresy is a going astray."  
This meaning is also apparent in many of the statements of the early scholars. For example, Sayyiduna 'Abdullaah Ibn Mas'ood رضي الله عنه said, "For one to be moderate in practicing the sunnah is better than for him to exert himself in heresies." Imaam Muhammad Ibn Shihaab az-Zuhri رحمه الله said, "Our scholars who have passed on would say, 'Adhering to the sunnah is salvation.'" Some of the earlier scholars take this definition even further and use the word "sunnah" to apply to all of the guidance that the Prophet صلى الله عليه وسلم brought and passed on to his Companions رضي الله عنهم and them to their followers, whether related to the foundations of the religion or specific issues. This is a very broad definition. Sunnah is the straight path, with all of its general aspects and detailed points. In fact, in this usage, the word "sunnah" becomes equivalent to Islaam itself. This is the meaning of the word "sunnah" as found in quotes from many of the early scholars. For example, Sayyiduna Abu Bakr رضي الله عنه is quoted to have said, "The Sunnah is the firm handhold of Allaah." 
 Naasir al-Aql, Mafhoom Ahlus Sunnah wal-Jamaa'ah Ind Ahlus Sunnah wal Jamaa'ah (Riyadh: Daar al-Watan, n.d.), p. 42.
 As noted, this was a very common usage of the term by the third century. Many of the scholars wrote works on the beliefs and foundations of the faith and entitled their works, The Sunnah. For example, As-Sunnah by Imaam Ahmad Ibn Hanbal رحمه الله, As-Sunnah by Imaam Ibn Abi Asim رحمه الله, As-Sunnah by Imaam Ahmad Ibn Muhammad al-Khalaal رحمه الله, As-Sunnah by Imaam Abu Bakr Ibn al-Athram رحمه الله, As-Sunnah by Imaam 'Abdullaah Ibn Ahmad Ibn Hanbal رحمه الله, As-Sunnah by Imaam Muhammad Ibn Nasr al-Marwazi رحمه الله, As-Sunnah by Imaam al-Asaal رحمه الله, Sharhus-Sunnah by Imaam Ibn Abi Zamanain رحمه الله and Sareehus Sunnah by Imaam Abu Ja'far at-Tabari رحمه الله. These works are not collections of hadeeth but are all works dedicated to the discussion of beliefs and fundamental issues of creed. Cf., 'Uthmaan Ibn Hasan, Minhaj al-Istidlaal ala Masaa'il al-I'tiqaad Ind Ahlus Sunnah wal-Jamaa'ah (Riyadh: Maktabah ar-Rushd, 1992), vol. 1, pp. 31-32; Muhammad al-Hamad, 'Aqeedah Ahlus Sunnah wal-Jamaa'ah: Mafhoomuhaa, Khasaa'isuhaa, Khasaa'is Ahluhaa (Riyadh: Dar al-Watan, 1416 A.H.), p. 16.
 Quoted in Naasir al-Aql, Mafhoom Ahlus Sunnah wal-Jamaa'ah Ind Ahlus Sunnah wal Jamaa'ah (Riyadh: Daar al-Watan, n.d.), p. 46.
 Cf., Naasir al-Aql, Mabaahith fee 'Aqeedah Ahlus Sunnah wal-Jamaa'ah wa Muwaqif al-Harakaat al-Islaamiyyah al-Muaasirah Minha (Riyadh: Daar al-Watan, n.d.), p. 13. This usage mostly started after the time of the Companions and Followers رضي الله عنهم. However, some quotes exist to imply that the earliest generations also used the term in this sense. For example, Sayyiduna 'Abullaah Ibn 'Umar رضي الله عنهما is quoted as saying, "Whoever leaves the sunnah has committed a blasphemy." See al-Aql, Mafhoom, pp. 42-44.
 See also: www.muftisays.com/forums/76-the-true-salaf-as-saliheen/56...
 This distinction was, obviously, not made until heretical beliefs and groups appeared in Islaam. In particular, it was used after the killing of the third caliph and companion Sayyiduna 'Uthmaan Ibn 'Affaan رضي الله عنه, during which time the Muslim community started to divide into sects. Imaam Muhammad Ibn Seereen's رحمه الله statement is well-known, "They did not use to ask for the chain of authorities [for reports]. But when the fitnah [the killing of Sayyiduna 'Uthmaan رضي الله عنه and the subsequent civil war] occurred, they would say, 'State the names of your men [meaning sources].' They would look for the people of the sunnah and take their hadeeth. And they would look for the people of heresy and not take their hadeeth." Imaam Ibn Seereen's رحمه الله statement is found with its complete chain in Imaam Muslim's رحمه الله introduction to his Saheeh. Cf., Imaam Yahya an-Nawawi رحمه الله, Sharh Saheeh Muslim (Beirut: Daar al-Ma'rifah, 1996), vol. 2, p. 44.
 Imaam Abul Qaasim Ismaa'eel al-Asbahaani رحمه الله, al-Hujjah fee Bayaan al-Muhijjah wa Sharh 'Aqeedah Ahlus Sunnah (Riyadh: Daar ar-Raayah, 1990), vol. 2, pp. 384-385.
 Recorded by Imaam al-Bukhaari رحمه الله and Imaam Muslim رحمه الله
 This hadeeth is saheeh. It was recorded, with slightly different wordings, by Imaam Ahmad رحمه الله, Imaam Abu Dawood رحمه الله, Imaam at-Tirmidhi رحمه الله, Imaam Ibn Hibbaaan رحمه الله, Imaam Ibn Abi Aasim رحمه الله, Imaam al-Baihaqi رحمه الله, Imaam al-Haakim رحمه الله and a number of others from al-Waleed Ibn Muslim رحمه الله from Thaur Ibn Yazeed رحمه الله from Khaalid Ibn Madaan رحمه الله from both Abdur Rahmaan Ibn Amr as-Sulami رحمه الله and Hujr Ibn Hujr al-Kalaai رحمه الله from Sayyiduna al-Irbaad Ibn Saariyah رضي الله عنه. This chain is of saheeh quality as all of the narrators are well-known trustworthy narrators.
 For a detailed look at this hadeeth, see: www.muftisays.com/blog/Seifeddine-M/3734_08-08-2014/stick...
 The Authority and Importance of the Sunnah, pp. 22-26