Sunnah: Fiqh Or Hadees
They refer to the Quran and Hadith, not to prove the practice, but to determine the status of an act being Farz, wajib or nafal. For example, saying Takbeer e Tahrima is sunnah, now the Fuqah see if it is farz or not, and then they refer to the Quran or Hadith. Secondly, they refer to a hadith to determine the Kefiyat of a posture, for example, where the Prophet used to place his hands while standing. This is not an essential part of prayer. Thus, it is not sunnah.