Who was the first to compile the purified Hadith?

The compilation of the Prophetic Hadith (definition) went through stages, beginning with some Companions writing down what they memorized. Then, during the era of Umar ibn Abd al-Aziz, the Hadith of the Prophet, peace be upon him, was collected, intermingled with sayings of the Companions and fatwas of the Tabi'in. After that, some scholars isolated the Hadith of the Messenger of Allah, peace be upon him, into Musnads, such as the Musnad of Ubaydullah ibn Musa and Musaddad al-Basri. The first to compile exclusively authentic Hadith was Imam al-Bukhari.