Who was the first scholar to take an interest in authentic hadiths?

The first to author a book specifically dedicated to authentic hadiths was Imam Al-Bukhari, followed by Imam Muslim. Imam Malik had shown concern for authentic hadiths before them, but his book was not exclusively dedicated to them, as it contained mursal (missing a Companion in the chain) and munqati' (broken chain) narrations. As for the concern with authenticating and scrutinizing hadiths, it began with the Tabi'un (Successors), when the deviant sects emerged, promoting their innovations with fabricated hadiths.