Hadith
Historically, not all Muslims believed in the hadiths. For example, Mu'tazilites rejected the hadiths as the basis for Islamic law, while at the same time accepting the Sunnah and ijma. For Mu'tazilites, the basic argument for rejecting the hadiths was that "since its essence is transmission by individuals, [it] cannot be a sure avenue of our  knowledge about the Prophetic teaching unlike the Qur’an about whose transmission there is a universal unanimity among Muslims". Further, those who claim to obey the Qur'an alone while rejecting external sources such as the hadiths and the previous revelations are known as "Qur'anists".