The Messenger of Allaah (sallal-laahu-alayhi-wasallam) said:
”Indeed, the one who lives long amongst you will see a lot of differences, and beware of newly invented matters [in the religion] because it is misguidance. So whoever amongst you reaches that, then upon him to hold onto to my Sunnah and the Sunnah of the rightly guided khulafaa, and bite onto it with the molar teeth.” [Reported by Ahmad: 4/126; Abu Daawud 4607; Tirmidhee 2676; Ibn Maajah 43]
Imaam Ibn Ibn Rajab [rahimahullaah] said: In this hadeeth is [the] command that when splitting and differing occurs, [one should] adhere to the Sunnah of the Messenger [sallal-laahu-alayhi-wasallam] and the Sunnah of the khulafaa [radiyallaah-anhum] after him. The Sunnah is a path that should be adhered to and it includes that which the rightly guided khulafaa were upon in creed, actions and statements. This is the complete Sunnah.
[Jaami-ul Uloom Wal-Hikam: page: 249. Slightly paraphrased]