In all probability, the first person who in academic circles used officially the expression ahl al-sunnah wa al-jama'ah was Imam Abu Ja'far al-Tahawi rh (d. 321 AH/ 933 CE) who in the introduction to his treatise on Islamic creed or aqidah (al-'Aqidah al-Tahawiyyah) said that the book was an exposition of the creed of the ahl al-sunnah wa al-jama'ah in accordance with the understanding of Muslim jurists.

While commenting on the Qur'anic verse: "And as to those whose faces turn white (on the Day of Judgment), they shall be in Allah's mercy; in it they shall abide." (Al 'Imran, 107),

Abu al-Fida' Ibn Kathir (d. 774 AH/ 1372 CE) remarked that a companion Abdullah b. Abbas d. 68 AH/ 687 CE) said that the people implied in the said verse were (the people of) ahl al-sunnah wa al-jama'ah. However, it is very unlikely that the term or the maxim in question was known as such during the early days of Islam and that Abdullah b. Abbas , therefore, could make use of it in his lifetime. This is because there are no reports whatsoever that anybody ever employed it at the time of the Prophet's companions or afterwards till perhaps the end of the 3rd AH/ 9th CE or the early 4th AH/ 10th CE centuries soon after which using the maxim ahl al-sunnah wa al-jama'ah became a standard. What Abu al-Fida' Ibn Kathir rh did in his exegesis of the Qur'an, it appears, was that he in the spirit of his time and the latter's lexis interpreted, rather than narrated verbatim, the words of Abdullah b. Abbas who in point of fact said (the people of) ahl al-sunnah and not (the people of) ahl al-sunnah wa al-jama'ah.

This could be further corroborated by another eminent exegete, al-Qurtubi (d. 671 AH/ 1272 CE), who made reference to the same account of Abdullah b. Abbas in the context of elucidating the same Qur'anic verse, but maintained that Abdullah b. Abbas said that those people whose faces will turn white on the Day of Judgment will be (the people of) ahl al-sunnah rather than (the people of) ahl al-sunnah wa al-jama'ah..........
https://www.islamicity.org/5069/the-...0%2C4422791087