The creed (‘aqeedah) of Ahlus-Sunnah wal-Jamaa’ah was penned down by Imaam Ahmad ibn Hanbal (may Allaah’s mercy be upon him) in his great work Usoolus-Sunnah translated as The Foundations of the Sunnah.
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.
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).