Who named the Muslims Ahlus-Sunnah wa'l-Jamaa‘ah (the Sunni mainstream)?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Some people may think that there is a difference between the common name "Muslim" and other names that, when mentioned, pinpoint a specific feature with which a particular group is distinguished from others, like, for instance, Ahlul-Hadeeth, Ahlul-Athar, and Ahlus-Sunnah wal-Jamaa‘ah. The last name came to exist when religious innovations such as Tashayyu‘ (profession of Shiism) and the Qadariyyah and Khawaarij sects emerged at a time near the end of the era of the Companions . The noble companion, ‘Abdullaah Ibn ‘Abbaas was the first to use the term Ahlus-Sunnah wal-Jamaa‘ah within his interpretation of the verse wherein Allaah The Almighty Says (what means): {On the Day [some] faces will turn white and [some] faces will turn black.} [Quran 3:106] 

He said that those whose faces will turn white are the followers of the Sunnah, and those whose faces will turn black are the people of religious innovations.

This expression was meant to outline the greatest features that distinguish the followers of the Sunnah, which are their adherence to the way of the Prophet , in word and deed; holding fast to the Muslim community and never being divided; and never disobeying anyone whose obedience has been made obligatory on them. These features distinguished the followers of the Sunnah from the Khawaarij sect, whereas the other sects mostly differed with the followers of the Sunnah in the first feature.

Allaah Knows best.