Shaykhul Islaam ibn Taymiyyah (:rahim:) said:

“The Sunnah, that is a must to be followed for which one is praised upon following it and blamed for going against it, is the Sunnah of the Messenger of Allaah (صلى الله عليه وسلم) in matters of beliefs, matters of worship and the rest of the affairs of the religion. And it is known only by knowledge of the hadeeth of the Prophet (صلى الله عليه وسلم) that are confirmed on his authority.”