The creed (‘aqeedah) of Ahlus-Sunnah wal-Jamaa’ah was penned down by ImaamAhmad ibn Hanbal (may Allaah’s mercy be upon him) in his great work Usoolus-Sunnah translated as The Foundations of the Sunnah. The term Sunnah in the title refers to the belief and the ‘aqeedah of Ahlus-Sunnah wal-Jamaa’ah. Indeed there were many works compiled in the early generations that dealt with clarifying and conveying the belief of the Companions of Allaah’s Messenger (salallaahu ‘alaihi wassallam), from them we have the following:
- Usool us-Sunnah of Imaam Ahmad ibn Hanbal (d. 241H)
- As-Sunnah of Al-Athram (d. 273H)
- Kitaab us-Sunnah of Abu Dawood (d. 275H)
- Kitaab us-Sunnah of Ibn Abee ‘Aasim (d. 287H)
- Kitaab us-Sunnah of Abdullaah ibn Ahmad ibn Hanbal (d. 290H)
- As-Sunnah of Al-Marwazee (d. 292H)
- Sareeh us-Sunnah of Ibn Jareer at-Tabaree (d. 310H)
- As-Sunnah of Al-Khallaal (d. 311H)
- Sharh us-Sunnah of Al-barbahaaree (d. 329H)
- As-Sunnah of Al-Assaal (d. 349H)
- As-Sunnah of At-Tabaraanee (d. 360H)
All these works and many more were compiled by the illustrious scholars of the past in explanation of the Islamic belief as it was understood and conveyed by the Companions of Allaah’s Messenger.
Those who adhere to the creed expounded in these works are known as Ahlus-Sunnah wal-Jamaa’ah and those who turn away and reject what these works speak of are known as Ahlul-Bid’ah wal-Furqah (the people of innovation and separation from the truth).
As for the book we are discussing, Usool us-Sunnah, then though it is small in size it is indeed great in meaning. Abu Ya’la al-Hanbali (died 526H) stated (Tabaqaat al-Hanaabilah 1/241):
If one was to travel to China in search of it – it would be no mean feat.
This aqeedah was penned down by the Imaam of Ahlus-Sunnah by consensus – the one who remained firm and steadfast in the times of tribulation in which the people of misguidance compelled the people with their saying that the Qur’aan was created along with the denial and distortion of the Attributes of Allaah. So Allaah, the Most High, made the truth clear by way of Imaam Ahmad (rahimahullaah), the Imaam of Ahlus-Sunnah – born in the year 161H and died in 241H.
This work begins with the chain of narration that connects this work to Imaam Ahmadibn Muhammad ibn Hanbal – and one should not doubt the blessing that has been conveyed upon this Ummah by Allaah by preserving this Religion by way of the chain of narration (i.e. the isnaad):
Ash-Shaikh al-Imaam Abul-Muzafar Abdul-Malik ibn ‘Ali ibn Muhammad al-Hamdaani said: Shaikh Abu Abdullaah Yahyah ibn al-Hasan ibnAhmad ibn al-Banaa narrated to us saying: My father, Abu ‘Ali al-Hasan ibn Ahmad ibn Abdullaah ibn al-Banaa, informed us saying: Abul-Husayn ‘Ali ibn Muhammad ibn Abdullaah ibn Bishraan al-Mu’dal informed us saying: ‘Uthmaan ibn Ahmad ibn as-Sammaak informed us saying: I read to Abu Muhammad al-Hasan ibn Abdul-Wahhaab ibn Abil-‘Anbar from his book in the month of Rabee’ al-Awwal in the year 293H and he said: Abu Ja’far Muhammad ibn Sulaymaan al-Munqaree al-Basree narrated to us in Tanees (a city in Egypt) and said: ‘Abdoos ibn Maalik al-‘Ataar informed me saying: I heard Abu Abdullaah Ahmad ibn Muhammad ibn Hanbal, rahimahullaah, say:
The foundations of the Sunnah with us are: Holding fast to what the Companions of Allaah’s Messenger (salallaahu ýalaihi wassallam) were upon.
The Messenger (sallallaahu alaihi wassallam) said:
Binding upon you is my Sunnah and the Sunnah of the Rightly Guided Caliphs after me – hold onto it with your molar teeth. 
And holding on to whatever the Companions were upon is holding on to the Book of Allaah and the Sunnah of the Messenger and to his Seerah (i.e. his biography) and to his Rightly Guided Successors, from the noble Companions.
The Prophet (salallaahu ‘alaihi wassallam) described the sect of salvation in the hadeeth of the splitting of the Ummah:
This Ummah will divide into seventy-three sects, all of them will be in the Hellfire except for one. He was asked: “Who are they (the Saved Sect)?” He said: They are those who are upon that which I and my Companions are upon. 
So if we know this then it is upon the seeker of knowledge that he read the biography and life of the Prophet and likewise about the lives of his Companions and how they were for indeed the innovations occurred after the time of the Messenger (salallaahu ‘alaihi wassallam) and the Companions opposed all innovations.
And the first of the innovations to befall the Ummah was the innovation of the Khawaarij , Then there was the bid’ah of ar-Rafd (i.e. the Shee’ah) , Then came the bid’ah of al-I’tizaal , And there also occurred the bid’ah of Al-Qadr .
So all of this was taking place in that time, yet the Companions withheld and never entered into innovations, rather they forbade them and opposed them as occurs in the hadeeth of Abdullaah ibn ‘Umar (radhi Allaahu ‘anhumaa) in Saheeh Muslim when he was informed of the bid’ah of the Qadariyyah so he said to those who conveyed that to him:
If you meet them, then inform that I am indeed free of them and that they are free of me.
Then he narrated the hadeeth from his father in which Jibreel came to the Prophet (salallaahu ‘alaihi wassallam) to him about the Religion. 
And likewise ‘Ali ibn Abee Taalib who took a position against the Khawaarij and sent his cousin, Abdullaah ibn Abbaas, to them and he debated with them after which four thousand returned to the truth and the rest remained, so ‘Ali fought them on the day of Nahrawaan. So we see the Companions of Allaah’s Messenger withheld from bid’ahwhich became apparent in their times – and they took positions of enmity towardsbid’ah. And that which clarifies their hatred of the people of bid’ah is the narration of Abu Umaamah al-Baahilee, the noble Companion who settled and died in Shaam in the year 86H, that is reported by Abu Dawood (died 275H) in ‘Kitaab as-Sunnah’ – that when Abu Umaamah saw the severed heads of the leaders of the Khawaarij erected upon stakes he stated that they are the dogs of the Hellfire and the most evil of the creation.
Footnotes Reported by Ahmad in his Musnad (4/126, 127); Abu Dawood in Kitaabus-Sunnah, Chapter: In Adhering to the Sunnah 4/200, 201 (4607); At-Tirmidhee in Kitaabul-‘Ilm, Chapter: That which has been reported regarded taking from the Sunnah and abandonment ofinnovations 5/43 (2676), and he said concerning this hadeeth: Hasan Saheeh; Ibn Maajah in al-Muqadimah, Chapter: Sunnatul-Khulafaa ar-Raashideen al-Mahdiyeen 1/15, 16, 17 (42,43,44); Authenticated by al-Albaanee in Saheeh Sunan Ibn Maajah 1/13 (40, 41, 42); And ad-Daarimee in his Sunan, Chapter: Ittibaa’ as-Sunnah 1/57 (95). Reported by Imaam Ahmad in his Musnad 3/229 (8404); Ibn Maajah in Kitaab al-Fitan, Chapter: The Splitting of the Ummah 2/1322 (3992, 3993); Abu Dawood in Kitaab as-Sunnah, Chapter: The Explanation of the Sunnah 4/197 (4596); At-Tirmidhee in Kitaab al-Imaan, Chapter: That which has been reported concerning the Splitting of the Ummah 5/25 (2640); and likewise this hadeeth has been reported by Ibn Abee ‘Aasim in Kitaab as-Sunnah 1/32,33, from chains of narration from ‘Awf ibn Maalik, Anas, Mu’aawiyah, Abu Hurairah and Abu Umaamah. Authentication by Al-Albaanee in his notes on Kitaabus-Sunnah of Ibn Abee ‘Aasim, in which he said: “This hadeeth is clearly and absolutely saheeh because it has six other chains of narration from Anas and supporting narrations from a group of the Sahaabah.”  They were called Khawaarij due to their khurooj (rebelling) against ‘Ali ibn Abee Taalib (radhi Allaahu ‘anhu) on the day when two Companionswere appointed to judge between some differing that had occurred, so they despised the judgment and the arbitration, and they said, “The is no judgment except for Allaah” – so they exited his authority. They settled in a place called Harooraa, so they were also labelled, for that reason, as the Harooriyah, and they have other titles that they were known by such as: Muhakkimah due to their denial of arbitration and their saying: “There is no judgment except for Allaah”; they are also known as al-Maariqah (the renegades or deserters) due to their leaving the Religion just as an arrow leaves the bow as occurs in an authentic hadeeth regarding them; they are known also as the Nawaasib (the plural of Naasib), and a person is called a Naasibee due to his extreme hatred of ‘Ali ibn Abee Taalib (radhi Allaahu ‘anhu).  Ar-Rafd means to abandon and reject. And they rejected the Imaamship of the two Shaikhs: Abu Bakr and ‘Umar (radhi Allaahu ‘anhumaa). They freed themselves from them both and they criticised and cursed the Companions of Allaah’s Messenger. See the work ‘Badhal al-Majhood fee Ithbaati Mushaabahatir-Raafidah lil-Yahood’ 1/850. They are known as the Raafidah and the Shee’ah, the Ithnaa ‘Ashariyyah (the Twelvers), the Ja’farees.  They are the Mu’tazilah. They were the followers of Waasil ibn ‘Ataa who abandoned and broke away from the gatherings of the great Imaam al-Hasan al-Basree (died 110H). They believed that the one who commits a major sin is not a believer or an unbeliever, rather he is between two positions and in the Hereafter he will be cast into the Hellfire. They have many other deviations besides. See ‘Al-Milal wan-Nihal’ 1/38.  They are the Qadariyyah. This was the title given by Ahlus-Sunnah to those who claimed they were creators of their own actions free of Allaah the Mighty and Majestic. They denied the intercession of the Prophet (salallaahu ‘alaihi wassallam) for the major sinners to be removed from the Fire; they denied that the beloved servants of Allaah will see their Lord in the Hereafter; they denied the Punishment of the Grave, the questioning of the two Angels, Munkar and Nakeer, in the grave, and the setting up of the Scales on the Day of Resurrection; they claimed that the Qurýaan was created and other than that from deviations. And there occur some narrations describing their characteristics and stating that they are the Magians (fire-worshippers) of this Ummah. And they share much with the Mu’tazilah in their deviation.  Reported by Muslim in Kitaab al-Imaan, Chapter: A clarification ofImaan, Islaam and Ihsaan and the obligation of having Imaan in the Decree of Allaah, the Perfect and Most High 1/36 (8).