Qadariyah (or Qadariya), in Islam, are adherents of the doctrine of free will. The word Qadar is derived from qadr (power or rights).
Qadariya was one of the earliest philosophical schools of thought in Islam. The doctrine espoused notions of rationalism and contained elements of Greek philosophy. Qadaris maintain that Allah gave man free will, without which one cannot be fully accountable for one's actions. Free will also means that Allah cannot know a man's actions in advance. Qadaris also deny other core tenants of Sunni belief including the belief in the Punishment of the Grave. They also deny that an authentic hadith is an evidence for establishing a proposition in the Islamic aqeedah unless it is transmitted in mutawatir form.
The idea of Qadariyah, i.e. the Doctrine of Free-will, came from a Persian named "Sinbuya Asvāri" who was put to death by the Umayyad Caliph Abdu'l-Malik, or, according to other narratives, by Hajjaj bin Yusuf. His idea was already taught in Damascus at the end of the seventh century of our era by Ma'bad al-Juhani (died in A.D. 699), who had imbibed the doctrine from Sinbuya.