al-Sunnah wa’l-Jamaa’ah are those who adhere to the Sunnah and who unite
upon it, not turning to anything else, whether that be in matters of
belief (‘aqeedah) or matters of actions which are subject to shar’i
rulings. Hence they are called Ahl al-Sunnah because they adhere to
it (the Sunnah), and they called Ahl al-Jamaa’ah because they are united
(mujtami’oon) in following it.
If you examine the followers of bid’ah (innovation),
you will find that they differ concerning that which they are following,
with regard to beliefs, methodology and practices, which indicates that
their being far removed from the Sunnah is commensurate with the extent
to which they have introduced innovations.
Majmoo’ Fataawa wa Rasaa’il Fadeelat al-Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him), vol. 1. p. 37